The most basic answer I have, since I'm not smart enough otherwise, is is your clutch cutoff button stuck on the loader? Is that what you are calling the shuttle push button I'm assuming that's the one on the gear shift lever? I don't know if I have the correct name for it.
On my NH lb75b the button on the loader lever gets stuck pretty regularly. The machine will rev but not go anywhere. I've got to kind of press it and slide my thumb of to the side to get it unstuck, like the plastic is just binding up or something. I could try a little lube in there maybe...