The answer is very simple: it's not that captains got weaker... there are just stronger people around!
The hype for the captains has finished.... a new hype has born. When SS arc started Shinigami Captains were presented as the most powerful spiritual beings ever.... now they are just ... well weak. That's how Shounen works and Bleach is a perfect example of the shounen formula.
Hype charachters and the humiliate them to hype the new charachters.
In no order but these people are supposed to be stronger than SS captains:
Aizen & co,Vaizards, Espada and every Arrancar made of more than a Gillian....
and THEN we have captains... they had problems dealing with Gillian-made Arrancar!!!