shifting

My Yardman 12 HP tractor does not respond when I shift it into gear. What can I look for as the problem

Hi, I bought a used Poulan Pro Riding lawn mower (model PP11536LMB) and am having some trouble with shifting. It shifts just fine when the engine is off and the clutch is not pressed, but when I engage the clutch, the shifter will not move. Any help is appreciated, thanks.

I bought a used Craftsman T 1200 247203720 and I'm having a hard time getting it to shift from forward to reverse. Once I get it into reverse, it works fine. I'm guessing it's an issue with the transmission linkage adjustment but, I don't want to mess with it until I talk to someone.