This is not at all uncommon on military surplus Mausers.
The problem is wear, mis-matched or replaced parts, or parts altered by a previous user.
What's usually going on is wear or mismatching of parts in the sear, or the cocking piece has allowed the cocking piece to sit too far forward.
The safety must slightly cam the cocking piece backward in the Mauser design to keep the cocking piece from resting on the sear. If this was allowed, the gun might fire if while on safe the trigger was pulled, then the safety was flipped off. For this reason, the safety's cam surface cams the cocking piece slightly backward.
If the sear, (the box-like part that wraps around the trigger inside the stock) or the cocking piece ledge are mis-matched, worn, or have been altered, the cocking piece will sit too far forward. The safety's cam surface won't rotate down in front of the cocking piece, usually hitting the top of the piece.
In order to work, the safety's cam area must rotate down IN FRONT of the cocking piece. What's probably happening, is the safety is hitting the top of the piece, not the top front edge, and isn't able to push it backward.
If the front of the cocking piece isn't too far forward, a VERY SLIGHT amount of stoning to bevel the top front edge of the cocking piece may restore it's operation.
DO NOT stone much, since you will quickly break through the cocking piece's case hardening, and this will ruin the part. DO NOT stone the safety, at all.
If a small bevel won't fix the problem, you'll have to try a different cocking piece, and/or a new sear box until you find a combination that will work.
