That's been my experience too. I routinely revive gummed-up Shimano trigger shifters at work. I wouldn't cast any aspersions on t.del for asking his question, because I've worked in shops through the entire era of Shimano indexed mountain shifters and don't remember seeing that failure mode myself.

t.del, if it doesn't respond to being de-gummed with WD-40, and there's no obvious debris jamming things up, you may just need a new shifter or brake/shift assembly, like some of the guys mentioned. But one other possibility to look for real quick, is if the "piano wire" strands that make up the cable housing have sneaked through the end-cap on the cable housing, and are intruding into the shifter along with the inner cable. I'm really reaching with that one, but it would only take a minute to verify. If you already have the housing out of the picture, then that's not it.