i had this once with a motox bike. as you are fitting a second hand clutch are you sure all the washers to seperate the outer basket and inner basket are fitted in the correct place. i found that one had been overlooked when someone rebuilt the clutch so the outer and inner clutch ended up essentialy bolted together.
if you remove all the plates but leave the basket and inner in place you should find they can be moved seperately. if not its almost certainly your issue