this happens a lot, there was just a thread about it too. Engage and disengage the pto. apply very gentle pressure to the lever, and either just as it stops rotating, or after its stops, the lever will move. Its just the gears not meeting up.
Most importantly don't try to force it, the linkages can get bent easily. You just have to get the gears to align and it will move easily. You can do that by manually moving the PTO shaft, or by engaging the PTO itself while applying slight pressure to the selection lever.