I've never completed that series. I'm not even sure, I've ever finished the first one in the series :( And with very little info here, I don't think I can be of any more help.
The only thing I can think of in general, is to check your logs, when you finished the last quest, and see if there was some bracketed "[]" dialog you didn't follow, and try that out. And check your /con level, to make sure it hasn't been lowered since doing the previous ones for some reason.
Good luck, and hopefully someone who has done it knows more :)
Yther Ore.