As stated, the detent is there to prevent unwanted movement when firing, handling, etc. It is subject to tuning, with care. Sometimes the detent is deepened to hold better. Some can be loosened by polishing the tip of the plunger pin and the surface it rides on (front edge of the thumb safety). The bottom edge of the dimple can be tapered downward with a jewelers file to loosen the hold. A drop of oil can't hurt either.
Check that the plunger tube has not worked loose before having any tuning done.