This bike is over 17 years old and not particularly high quality, but it does not make sense to replace the whole thing over one shifter. So I am looking to repair it if possible. I'm confident with most aspects of maintenance, but I've no idea what goes on inside these things.
When changing 'down' to an easier gear with the thumb, the lever slips and the gears do not change. The lever moves further than is expected, with no resistance. By simultaneously applying lots of upwards pressure with the thumb, you can sometimes get it to shift. Once it does shift, the rear functions happily. I've had no other derallier issues.
There's an allen key fitting on the top, and a screw on the bottom. I have not opened it up, worrying that springs will ping out and all will be lost!
I've no idea if it needs more lubrication... or all the gunk cleaning out. Go tighter... or looser?