Strange Mouse/Mixture Problem

I have always used the mouse to control the mixture since my Ch Flight Sim yoke doesn't have a mixture control. Now for some reason the mouse will only control the mixture on some airplanes. Doesn't matter if it is a default or add-on plane. I can still control the mixture using the keyboard but I have never done that before. The mouse (acutally a trackball) will control the throttles and prop controls ok on everything-just not the mixture. On some planes it works fine; on others it will not move the mixture control. I've even tried another mouse with the same result. I still have FS2002 on my system and the mouse will control the mixture on everything there. The only thing I have done recently is to replace my CRT monitor with a LCD type and I have added more memory. I am at a loss as to what is going on here. Any ideas from anyone?.Jim