It's harder to go down and depending on position of application of force it may also do a weird sound indicating it 'clamps' at some point [this is especially irritating since it feels like it's 'glued'].
Any information that might help?
Mice tend to get buildups of gunk inside them because most people eat and drink at their computers. You have two choices: buy a new mouse or disassemble the mouse and clean it. The latter is worth a shot, since you really don't have much to lose. eHow's basic instructions for disassembling a mouse may be helpful. Unfortunately, the microswitches most commonly used for mouse buttons are both very small and typically "sealed" units (i.e. fully enclosed and not meant to come apart). But you should be able to spot them easily enough. I've had some success just bathing the things in alcohol applied with a QTip and then waiting for them to dry out. Good luck!
I fixed this with some hackery: Under the right click (it requires 'open Operation') there's a plastic, sort of "clamp" which appears to be getting in a hole of some sort acting as a kind of stabilization mechanism. (sorry for the english)
The point is if you just cut it off, it fixes.
I had initially thought of "Filing"(as in a 'Fingernail File') edges of contact of the key and rest of mouse around it but that did nothing.
There's no noticeable stabilization problems with it.
EDIT: It may still need a "prop" of some sort under the key where it contacts the actual 'clicker' (e.g. with duct tape (small pieces)). It's not extremely easy but not impossible. It's stable for at least a month now.