your LW level has exceeded your skinning level and mob level.
thick leather/hide usually will be found on level 45+ mobs. you'll, of course, get the occasional thick from lower levels but just gain a few levels and you can get the thick stuff.
It would probably be most efficient to grind/skin some lower level mobs (try the nightbane in duskwood) then to AH the lower leather/hides and use the gold to buy your thick hides