The button is not any real type of forward assist. If the bolt is stuck open there is a good chance that there's some type of lead or plastic debris inside of there holding it open, or something like a spring is broken and lodged holding it open. There bolt has a long spring that operates it, and it works (more or less) on momentum when everything is fee and clear to move the way it was designed.
Take a close look at the magazine. You may find that there is pieces of the inner gears broken off. The probe smashing the magazine is common if the magazine isn't cycling as quickly as it should based on bolt timing. This slow cycling/timing has a number of causes, including heavy long pellets or slugs, or if some pellets were loaded up backwards, or going too far beyond the system parameters while "tuning". Slow cycling can also smash the corner of a pellet's skirt off, sending this debris backwards into the receiver.
Before tearing it down, try holding the charging handle all the way back, pointing the muzzle down, while blowing compressed air around the inner areas to dislodge any lead or plastic debris.
Also try pulling the charging handle all the way back and pull the trigger to release the sear and de-cock it. Something may have locked things with the trigger/sear engagement mechanisms.
