You gotta finesse it...
Getting and keeping an agro is an art... well... an art that becomes so repetative we sometimes forget it.
Agro is generated in 3 main ways. Doing damage to the mob, doing malicious effects/debuffs to the mob, or healing someone who has agro from the mob. This agro is a numbers game, pure and simple.
Now this agro is factored into who the mob attacks in a pretty simple way. The mob finds who is in attack range, then finds who is on the agro list out of attack range. It then calculates whether or not it considers it 'worth it' to chase down someone with higher agro out of range or to stay put and fight.
There are also some exceptions to the rules in this number game.
1) Root, as stated, will make the mob attack the closest PC it can find, followed by the closest enemy NPC it will find that has agro (such as a pet).
2) Charm (when it breaks) will automatically put the charmer at the top of its list by a certain percent. The total hate on the mob can effect how many hate points that % is, so this is why pulling agro off charm can vary.
3) 'Taunt' works as a % modifier, not a hate point addition. When you do not have agro on the mob, a sucessful high-skilled taunt WILL put the taunter automatically at the top of the list. A person can be engaged building agro for hours, have a Paladin come in and taunt 1 time, he now has agro (by a very small amount, possibly only 1 point.)
4) Event agro (started in Velious expac and found throughout the game since) does not follow these rules. This agro generally works in clusters, and mobs will agro due to general location and build their own agro on these targets which is hard to break. (Examples - Ring War in Great Divide, High Priest of Ssra adds, Butterflies in Terris Thule Event in PoNB) I wont talk about this now its too different and I still don't understand it sometimes.
Using what I've said we can pin down your problems.
-Your problem with this ranger is plain and simple: First this ranger at this level is undoubtably outdamaging you. Using taunt will get you a tiny bit of agro over HIM for a tiny bit of time, until you let him hit again and start doing more damage. To add to this big problem is the fact that this ranger is most likely using Snare. Snare falls into the second way to get agro, and boy this is a doosy. He may also be using lowbie DOTs which don't add much agro through the DPS, but the debuff factors in separatly and can also be nasty to break. Finally since the ranger remains in melee range, the mob chooses him as the obvious target.
-- To fix this problem you need to get a flow down. Always keep in mind anyone who is using debuffs and curses (including DOTs) on your enemies. In the case of this ranger, he is most likely starting off with his debuffs/snare early. This is good for you, since you can secure agro right after this and maintain it the duration of the fight. Heres how...
You will need to get out your Cease and Desist spells onto your spell bar. They are each almost as much agro as the ranger's snare and will be your greatest ally in the fight for agro.
If your not pulling, as soon as you can get the mob targeted start casting your cease. Remember not to do it immediately if theres a chance it will somehow ***** up the pull, and if your group is annoyed at the stun portion practice timing it to land as the mob arrives at the battlefloor (you wont have to worry about this end game when Cease/Desist stop working).
Now you might already have agro, but if hes pulling with snare or a DOT you probobly wont. Also he may not be done yet, so dont use your taunt randomly.
To get the max effectiveness from your taunt and the rest of your spells, your best bet is to attempt to get the % jump up using taunt first, following it immediately by another stun (preferably cease again). What this does is jump your agro up the scale first (sometimes to the top by just a tiny bit, once you have higher skill it will always do this), then the stun will boost your agro numerically giving you a buffer in hate points.
Example - {P = Pally, R = Ranger} - Guestimations in Hate
Wrong way...
R pulls with Snare, 560 Hate
R runs back and begins casting DOT, 340 Hate, 120 Hate/tick
P stuns, 480 Hate
P disarms, 200 Hate
P taunts, rolls for 75%, 165 Hate
... Ranger still has agro by 55, and his DOT + DPS ends up continueing to outagro the Paladin.
Right way...
R pulls with Snare, 560 Hate
R runs back and begins casting DOT, 340 Hate, 120 Hate/tick
P taunts, rolls for 75%, 675 Hate
P stuns, 480 Hate
P disarms, 200 Hate
... Paladin ends up on top by 455 Hate. Not a whole lot, but definatly enough to keep agro. Now if the Paladin continues to use disarm and stuns, as well as throwing in taunts for extra boosts I assure you no one can break your agro for more than a second (by using your own tricks against you, such as taunt.)
If taunt fails you will encounter some problems. But just keep throwing on those baby stuns. Once you get enough agro on these guys you shouldn't need to keep chaining anyways, and you can give your mana a small break which may be required at that level.
High end there is no class** that can reliably outagro a Paladin without using their own purposefully attention grabbing tricks (Wizzy Snare + Debuff, Charms, atones, roots, and intentinally agressive DOTs). Even then, the Paladin can usually regain agro through the Taunt + Stun technique. People have argued this with me on multiple occasions but I have yet to find people who could keep agro off me for more than a tick because of stuns, taunt, and a few other tricks up the Paladin's sleeve.
**The only problem I have ever encountered in a true agro battle is with a pet-less SK or Necromancer. Since they can do a DOT + Snare in 1 which can be chained and piled with more DOT dps which can be used to outagro your attempts to gain agro after a taunt. This isn't a problem unless they are trying to do this, however, and so necromancers and SKs are hardly ever a problem.**
As I said, there are certain agros in the game that have different agro calculators, and I'm still not sure how they all work but in most grinding situations this wont effect you.