Mower - Autodrive poor after belt change

I've just replaced the belts on my Hayter 41, and I've now got very poor drive. The roller turns but has what I can only describe as 'little toque'. I've maxed out on the adjustment of the clutch cable.

What else could be causing this, I appreciate that I may have put it back together incorrectly, if I have then I'm not certain where to start looking. Any tips greatly appreciated.

Check the new belt.
I had this,on a cutting belt not drive, and I'd bought the correct "spec" of belt but it didn't work, too loose. So I bought another, different, correct spec belt and it was visibly shorter. All was well after that.
If you have a new, maufacturers belt then I'd still try another, if a pattern part definitely try another.