Ultegra 9-speed brifter upshift problem - beyond WD-40?
This link may be useful to you if you really want to dig around inside your shifter; though it looks like it might require a few special Shimano tools. Also note that he only has full strip/rebuild info for Dura-Ace...though I would expect Ultegra to be substantially the same, differing mainly in the use of plastic instead of metal for some parts.

Also consider that it is pretty easy to find Ultegra 9 stuff on eBay and elsewhere, and it may be more time-efficient to just replace rather than repair.