I just recently got a PTR 91, and thought I'd try some cast with GC.  They shoot dandy out of my old converted MAS, but it was hit and miss out of the PTR.  I got about a 70% reliability, with the brass ejecting as far as my ball nato rounds, it would just FTF some with the cast.  I was using a 200gr lyman GC over 30 grains of IMR 4350 which yielded 1984fps.....  

If you are getting reliable feeding, could you share a recipe?

Not entirely up on my acronyms:  Failure to feed (my guess) or failure to fire?  I would guess bullet shape is the culprit as 7.62 NATO tends to be pretty long.  You could play with bullet seating depth and maybe make some headway.  Not really enough info to go on.