One problem with surplus Mauser's is replaced or worn parts can cause a condition where you can't get the safety on, or it takes force to get it on.
The problem is caused by mis-matched or worn parts allowing the cocking piece to sit too far forward.
The cam on the safety simply can't get between the cocking piece and the bolt shroud to cam the cocking piece back.
The "fix" is to interchange the sear block and cocking piece until you find a combination that holds the cocking piece back far enough, or to SLIGHTLY stone the cam surface on the cocking piece to allow passage of the safety.
You have to be careful stoning the cocking piece, since the parts are case hardened, and anything over a very slight amount will break through the case coating, and ruin the part.