It could be a couple of things - I know you said it had enough fluid - how do you know/how did you check? Just trying to make sure - I changed my primary/transmission to Amsoil, and it made a HUGE improvement in the shift feel.
If it's something internally, it could be a couple different things - easiest is to check the clutch adjustment. After that, it could be the shift pawls out of adjustment. There may be a few other things, but I'll leave that to the experts.
-Brad