It's the cam sprocket that needs to be free to rotate,
not the IP sprocket. This means loosening the cam sprocket bolt a turn or two (using a sprocket locking tool, not the cam lock itself) and then tapping on the sprocket (or using a puller) until it breaks free of the tapered joint and can rotate independent of the camshaft
Kinda a pain, but needs to be done... otherwise timing belt slack has nowhere to go and you end up with timing issues exactly as described.
You can cheat by playing games and moving the IP sprocket back and forth by a tooth, but the cam timing will still be off (VCDS cannot show you cam timing), power will suffer, and there will be lots of white smoke. Sounds familiar, I'm thinking?
There's a great "A4 Timing Belt" pdf here that has great step-by-step instructions, with pictures at every step...might be worth tracking down if you would benefit from a more detailed explanation.