What could cause extremely high effort to release a Tost CG hook on aTwin Astir?

The scenario is winch rope break practice with the instructor pulling the release to simulate rope breaks.

I'm not talking about just an ordinary hard pull. This is a maximum-effort, adrenaline pumped, two-handed strain to get the release to work.

On the ground, the CG hook checks out as a little stiffer than one might prefer but otherwise normal. But trying for a mid-winch launch release it requires all the strength I can muster using both hands. Tost CG hook pull effort is not supposed to be dependent on rope tension.

The pull cable runs and pulleys have been checked and re-checked. The Tost CG hook itself is new and seems to work as advertised. There is nothing obvious one can see with the seat pans and instrument pedestal cover removed.

Any ideas?