Td5 Drivebelt Adjuster Issue

I hope someone can help?
My drivebelt looked a bit frayed so decided to change it. I released the adjuster pulley, took off the old belt & fitted the new one.
I then tried tightning up the adjuster pulley bolt what tightened up ok but it did not take up the slack. I then undid the bolt and moved the adjuster pulley tight using a bar & tightened the bolt hoping that would hold it but on releasing the bar it all springs back the the loose position. I tried a few times but it kept springing back to the loose position. Dont know what to try next.. The manual didnt say anything about this happeneing, just undo then do back up after fitting the new belt
Thanks in advance Ian

Arn't the pulleys tensioned by a spring? Surely you shouldn't need to loosen anything?

The pulley is tensioned by a spring mechanism, I just followed the instructions from Haynes manual but something does not seem right. The manual did say undo the pulley & too tighten it after fitting new belt but after its sprung to release the belt it will not now take up the slack when tightening back up.

Hmm. to fit a new belt to my tdi i just put a socket over the pulley nut, pulled it back against the spring, and pushed the belt on. Sounds a bit suspicious to me