I put a new drive belt on and for 2 days it shifted just fine. This morning I started it and let it warm up for a while and it starts grinding when I try and put it in gear. I thought maybe the idle was too high, but it still does it even with a low idle. It seems to shift ok with the motor off. I didn't check belt deflection when I put the new one on because I couldn't find a straight edge. If I have to add shims to the driven clutch, is this a big job?
It's just kind of a mystery that it would develop this problem from just sitting overnight.

 

Check your shifter linkages they could have loosened up or may need to be adjusted.