Anschütz Super Air 2002 SSP - trigger issue

If I cock the rifle normally, when the trigger is pulled the action just clicks and the 'cocking indicator' just moves forward a tiny bit.
If I push the cocking arm all the way forward until the action is cocked and the loading lever has moved back and then on the compression stroke stop the movement about half way and pull the trigger the action fires ok.

The rifle has been like this since I got it with a defective piston seal. 

On inspection all parts look ok and nothing is broken that can be seen.

Any thoughts on what the problem might be?