lawnboy transmission

I have a lawnboy model 10525, dura force 6.5, with 3 spped transmission, started have problems with trans. making a noise and then only 1 gear no 2 or 3 gear, took the trans apart and replaced the pinnion and ring gear ,the end bushings,the slider keys, to no avail still have the same problem.
any idea as to what to look for now?
thanks