I have had this problem, and it is really annoying when it comes up.  Occasionally, My joystick slider will stop controlling engine #2, and no matter how many times I press E+1+2, the engine will refuse to be controlled by my joystick. 

Please don't tell me to register FSUIPC and use the "Control all engines" command or whatever FSUIPC uses, because I paid $35 for the product, and I don't want to pay an extra $20 just to get it working properly.

a. What exactly doesn’t work or works wrong?

Occasionally, My joystick slider will stop controlling engine #2.  Engine #1 will respond, and the EICAS will respond to both engines (The green pointer on both engine readouts will move), but only engine #1 will change in thrust.

b. When it happens?

I try to resolve it with the FS ckey combination E+1+2, but nothing happens.  Once this issue starts, I am forced to start the whole flight over if I am at my departure airport or be forced to use my mouse, resulting in slow/difficult taxxi and landing.

c. How exactly do you think it supposed to work?

I should press E+1+2 and both engines should respond.

d. What is your system? (OS, MSFS version, video, RAM, CPU)

Windows XP SP2
FS2004 (9.1)
ATI Radeon X850 (256 MB)
1024 MB RAM
AMD Athlon 64