Sounds like you have a sticky clutch, which is a common problem for tractors that haven't had a lot of use. It's caused by rust on the pressure plate. The problem should go away as you use the the tractor. You can speed up the process by setting the parking brakes, putting the tractor in high gear and holding the clutch right at the point of engagement until it loosens up.
And yes, if you can't get the clutch free, it will require a tractor split and (expensive) clutch pack replacement.