Have you replaced the cable or is it the one fitted before changing the box?
If not that's where your problem will be as they have a funny self adjusting mechanism which only seem to adjust one way and not the other, also did you clip the cable back together (using the plastic lug's) before changing the box, because if not the cable would of gone past it's max adjustment and can't be re done (I found this out to my expense on a new cable and had to buy another)
Also if it's not a recon/new box it would be worth replacing the ratchet (behind the green end cap on the box) as these have an awful habbit of cracking leaving you clutchless and your clutch pedal on the floor.. they only cost a few quid from any vw spaecialist but can save you lot's of greif.......
