I don't know what would cause that. Did you try sliding the other valve down in the valve seat to see if it fit right and moved freely. If it does, I would just put a new valve in it and hope it doesn't do it again.
I've seen valves stick like that, but only if the engine has been sitting for a long time without starting it.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.