I've noticed recently that on heavily-used Wacom pens, especially those owned by a complete klutz such as myself, have a tendency to eventually get really short nibs. So short that it can only detect pressure when held completely perpendicular to the screen. At first you think, "Oh, wow I need to change the nib, that is seriously worn down, geez!" And when you change it out for a fresh one, it's JUST as short as before....
So what's wrong? To find out, give your pen a back-and-forth shake. Does it sound like the innards of the pen are shifting, rattling back and forth? Press on the eraser tip. Does the tip's length suddenly come back? If so, this means that the body of your pen has begun to come apart, thus making the shaft longer and loosening up the inside. (The following is a description of an Intuos4 pen. But others should be similar.)
To fix this, it's an easy matter of removing the duo-switch, (just get a fingernail underneath it and pry it, don't worry, it's strong,) then the cone-shaped grip holder by the tip, (twist it off,) then the rubber grip itself. If you hold the exposed plastic shaft where the grip used to be and the part closer to the eraser and gently (but firmly) pull apart the two halves, the pen should open up. !-Warning: The eraser tip has a small spring inside. Make sure it doesn't fall out! Also, Be careful not to damage the inductor coils at either end of the circuit board. They're held in place with very thin wires! Warning-!
(If by chance one does fall off, don't fret -- just put it back in place. The inductor coil by the tip should go at the VERY front -- the nib actually passes through it.) Inspect both halves of the shaft and the circuit board. If everything looks alright, drop the circuit board it back into the nib-side of the shaft first. The circuit board will only drop in one particular way, so make sure you have the two buttons for the duo-switch facing the right direction to line up with the holes in the shaft. Then, bring the eraser side of the shaft over the board and firmly press it back together with the nib-side. The board will also only fit a particular way into the eraser-shaft, so make sure you carefully align it first with the interior notches.
Now that everything is re-seated, the rattling should probably be gone from the pen when you shake it back and forth, and the nib should be at its full length once more. However you're probably not ready to draw with it again. Placing pressure will probably cause the two halves of the shaft to push apart again. Thus, before putting the rubber grip back on, take a very short cut of CLEAR PACKING TAPE and wrap it around the part where the two halves of the shaft meet. (Be careful not to make the length of tape so broad that it covers one of the duo-switch holes.) Wrap it around the shaft tight and mat it down to get maximum grip. Use just enough tape to go around the pen ONCE, you don't want to overdo it or else the rubber pen grip probably won't fit properly anymore. I suggest you use clear packing tape because it's very strong, and remarkably sticky to materials like plastic. Regular scotch tape will probably eventually let go, or break. I don't suggest using super glue on the off-chance that someday you will need to open the pen up again. Removing packing tape is a hell of a lot easier than dislodging super glue.
Now you can put the rubber grip back on, the grip holder, and the duo-switch. Most wacom pens should be like this, at least from Intuos-4/Bamboo onward. Here's a quick photo-guide on disassembling a Wacom Bamboo Pen.
Note that this is also a valid method to remove a STUCK nib in your pen. Let's say if the nib is physically too short and you can NOT remove it with the nib tool or anything else, disassembling the pen like this is probably the quickest and safest method to get it out.
Hope this helps a select few of you out there!