RON L here = SERESURPLUS
Well I'd have to agree with the good call of the previous poster! Check the firing pin, for the obstruction, Hell, Id take the bolt out, take that all apart (If you know how) and dunl the entire mess in some good cleaning solvent! It's possible you have enough gunk, grit, dirt and what not in thier to be stopping things from hitting as hard as it should?
If all the cleaning goes OK and you still have the Problem, time to start looking at replacement of the firing pin, the Mainspring, as well as inspection of all the fire control parts before you do any replacment as you might have grit and dirt fouling up the lower parts that are not letting the mainspring, make the hamer hit the primer hard enoug? The good news is, none of that is too hard to do, and can be fun and easy if you take it slow and consise?
Please do some/all of that and get back to us? It's not often an AK of any kind stops working? Let us know OK< WE wan to help and get your Mac-90 back shooting hard!@