what two speed is it? is the two peed a retro fit or build for yuor buggy.

most two speeds work on the principle of a centrifugal clutch, and if the clutch gets wet, or gets any oil on it it will not change gears, or will need to spin very fast to change, so make sure the gear set is dry, if you know what your doing it might be worth pulling the setup apart and checking the clutch.