You will need to go into more detail as far as what you are using and where you have the restriction located.
Photos "REALLY" help when trying to "SEE" what is going on and/or where the stopages are so that we can better help you.
Just from your short information.......it sounds like a couple (maybe-3) things are going on......
1: An SA42/semi-only-M53 firing full-auto sounds like either the bolt-assembly is recoiling back far enough to eject the spent-blanks/chamber the next blanks...."but" not far enough to fully cock the hammer of your guns semi-only design.........."or":
2: That you have a defective semi-only trigger-group (?)....."or":
3: That your firing-pin is either bent/broken and/or stuck in the bolt-head where it is sticking out of the face of the bolt firing anything that is chambered (??).
If the gun does not recoil the bolt-assembly far enough to have the hammer caught by the disconnector.....the fix is to use a slightly smaller BFA Hole-Size/Restriction (where-ever your gun is being restricted at.....????).
Next, AtlanticWallBlanks sells several different 8mm-Blanks. Are these the full-length 8x57mm Blanks (?).....or the shorter 8x57mm Blanks made from 30/06 Brass (??).....or the resized 7.62x51mm/.308-cal. surplus-blanks sold as 8mm Blanks (???). You can't always go by the case-head-markings as some vendors use re-formed 30/06 brass to make 8x57mm Blanks for reenactors..........again, PHOTOs really help us to better help you.
Questions that will come up are as you load your semi-only SA42...pull the bolt to the rear and chamber a blank (remember-semi-only....not MG42 full-auto):
1: You pull the trigger and exactly what happens (?).
2: What have you tried (?)......what BFA Hole-Sizes have you tried and the results (?).
Regards, RichardS in MI.
US Army, RetiredBlanksguy2001@chartermi.net