Broken / bent valves or the timing marks are wrong... did you replace the belt because the belt was broken?

Standard or automatic transmission?

What alignment marks did you use at the crankshaft? The camshaft mark is an obvious mark to align, but the crank can be tricky.

Thomas

1.8T, TDi or 2 liter?

Do you have spark? Injector Pulse?

If diesel, did you use all of the Alignment Tools? Was the camshaft sprocket loose when adjusting belt tension? Did you check Pump Timing with a Scan Tool?

Thomas