Ohh yea this was an issue i ran into bigtime with my trx bottom end!! Went through 3 center case gaskets. Mine was a locking washer that only slide over the grooves when it was spun correctly. I would take back apart make sure all your stuff is correct. Also put some oil on your tranny if you havent already. That thing should shift through the gears but the trans needs to have a steady move for it to seem smooth. Otherwise itll feel like it isnt shifting. So get a good rotation on the counter or drive shaft or even the crank. Hope this helps you some