Sounds like you havent adjusted the tensioner pulley, they arent set out of the box. There is a pdf floating around somewhere of how to do it, basically it needs set so that you can slide the small pin in and out before actually removing it.
Set pulley tension so that the pin is free
Tighten sprockets
Remove pin and tools and rotate at least 2 or 3 times by hand
Recheck tension (pin should still fit in hole) and if ok remove the pin, if not tweak the pulley tension and repeat
Think thats the right order lol
Apologies if that's what youve already done
http://www.themgzs.co.uk/vb/showthread.php?t=30124