Is it a selective or nonselective trigger system? I'm not familiar with the Mauser system but I've seen some older guns that the SNST system worked in a way that the second shot hammer didn't have quite the same travel. If it has one of those then a weak hammer spring could do it. Or It could be something in the selector system binding up slowing the second hammer, also make sure the gun is not too loose. Since it fires the top barrel first ok I doubt it is a firing pin problem. Pull the stock off and operate the triggers and it should be apparant whats causing it.
Ross
Ross