Me han respondido de airsoftsystems y efectivamente parece ser del cut off.
Dejo aquí su mensaje:
Hi Oscar,
the cut-off lever provided with the ASCU does not work well with some gearboxes, we are currently updating in in the mods, so this will be fixed in our next production.
The fastest way to solve the problem you have is to use your original cut-off lever, you will only have to file down the notch for the return spring.
We will be able to send you an updated cut-off lever next week.
Best Regards.
AS Support
A que se refiere con: you will only have to file down the notch for the return spring ( en caso de usar la original)
Muchas gracias a todos los que me estáis ayudando, espero que si a alguien le sucede esto quede resuelto y les sirva de ayuda.