well if the bolt carrier key is not loose and staked like it should be, there is nothing lodged in the ring area or bent rings on the bolt. my next guess would be the gas system on the upper.
there could be an obstruction in the gas port area or gas line.
and it could also be that del-ton is a bit conservative with the gas port size.
i generally don't open up a port till i have eliminated all other suspects though.
if the bolt carrier and bolt looks OK and functions in another rifle you have pretty much narrowed it down to the gas system on the upper.
also be aware that some russian made ammo is a bit --how would we say this? er underpowered.
not to long ago i got a bunch of tula 223 from CTD that wouldn't function in anything for more that a couple rounds before it would fail to cycle the action.
so while your trying various things try a portion of the ammo in another rifle that you know works.
also double check the lower and run it for a few rounds on a running upper just to make sure everything it right with the lower and nothing is binding with it.
you would be shocked at how many times a bind in a buffer system or LPK will cause a short stroke like FTF!
if you have covered all the bases--then you can move on to the gas system. the "dissy" system most of them use the 20" gas system.
this is generally OK for full pressure rounds but if the port is standard 20" size it is likely to have problems with low pressure russian ammo.
also be aware that some "dissys" run 16" CAR type gas systems with a bolt on gas block that fits under the hand-guards. these are generally better for low pressure ammo as the CAR short gas system better matches the low pressure russian ammo.
the bolt on gas block can be a little off the port --this is an easy fix as you can just straighten up the gas port and block --re-tighten and it should start running.
the 20" will probably have a front sight base that is pinned on. the pins will have to be pressed off. some are very tight and some are tapered. seems like del-ton are tapered. once off i would check the port alignment and probably open it up one notch from whatever size del-ton drilled it.