Gearbox issues help !!!

[Lloyd11]

I've got an 02 plate vw passat, it's a 2.5 v6 tdi auto. I have just changed auxiliary belt & tensioner. Obviously to do that I have had to take the front end of the car off. As for the radiator & fan everything went smoothly but as I've started it up & gone to drive it my gear box is playing up & feels like it is slipping in every gear. Does anyone know if I'm missing something or is this just a coincide? Thanks.