I don't think your tractor needs a clutch adjustment.
I don't think your tractor needs a mechanic, either.
I think your tractor needs cleaned & oiled; or rather, one part needs cleaned and oiled.
Grab a12mm socket, rachet handle & short extension (2 inches or so; a box-end wrench will probably work as well) Underneath the high/low lever, and maybe 1/2 inch forward of it's shaft, there is a bolt threaded into the side of the trans. Remove that bolt, and check to see if the lever moves more freely. Keep in mind that with the engine off the high/low may not fully engage but the lever should move enough to tell you if you have found your problem.
The high/low lever is locked in gear and neutral with an external detent. A spring-loaded plunger is located on the underside of the lever, and the plunger engages round holes in a steel plate on the side of the gear box. When you remove that bolt you allow the plate to pivot with the lever, essentially by-passing the detent.
If that appears to be the problem, you need to find a way to clean/oil that detent plunger. Removing the lever from its shaft makes that easier, but driving out the pin the holds the lever on may not be easy, working in the foot well.
When reinstalling the bolt, note that it's hole in the plate is elongated. Make sure the lever is aligned with the neutral slot in the plate on top of the trans when the detent plunger is in the neutral hole. Make sure the gears fully engage before operating the tractor.
