Usually that means the spring is jammed under the keycap. If so you're in luck, all you need to do is hold the spring in the middle of the hole (i.e. by lifting up the front of the keyboard) while you plop on the keycap. Good chance that will fix it.

Alternately, as it's a stabilized key, it could be the stabilizer getting rough. It might need a little bit of lube...

dionb wrote:

Usually that means the spring is jammed under the keycap. If so you're in luck, all you need to do is hold the spring in the middle of the hole (i.e. by lifting up the front of the keyboard) while you plop on the keycap. Good chance that will fix it.

Alternately, as it's a stabilized key, it could be the stabilizer getting rough. It might need a little bit of lube...

I assume now it's the latter. As I wrote in the edit, with a different unstabilized keycap, it depresses correctly. I read one shouldn't use synthetic oil based lubrication. What kind of grease would be ok?

Not sure I would use grease at all.

I would pop the stabilizer off and clean it and the hole it goes into. Chances are that there is just some dirt or fuzz in there and/or the plastic has worn a bit and has plastic dust in there. Either way, cleaning it should make it work fine again.

cyclone3d wrote:

Not sure I would use grease at all.

I would pop the stabilizer off and clean it and the hole it goes into. Chances are that there is just some dirt or fuzz in there and/or the plastic has worn a bit and has plastic dust in there. Either way, cleaning it should make it work fine again.

Ok, i will try that, too. Wonder if some pincers will get the stabilizer out...

Update: before doing anything with the keyboard I swapped out the numpad 0-key with the backspace key and noticed that the 0-key works just fine in the backspace position, whereas the backspace keycap is still sticky within the 0-numpad position. So it must have something to do with the keycap itself. I notice that the backspace key has a small ridge in the guide for the buckling spring. Maybe that makes a difference? Other than that both keycaps look identical.

Anyway, I ordered a replacement backspace keycap and a handful of replacement buckling springs, just to be sure, as well as one guide inset. So lets see if that will make a difference.