Mr. SS,
I am far from being the most knowledgeable on the internal operations of the 300 AAC Blackout in an AR/M4, but;
The lightest bullets I have loaded are 110gr V-Max and the heaviest were 220gr SPs. this is quite a weight range for any weapon, but to cycle an auto-loader....
My understanding is that the lighter bullets 'over gas' and the heavy bullets are on the light side in gas. As the heavies are normally for sub-sonic and use with a 'can', the uses of the 'can' pushes up the pressures (gas) and we have consistent cycling and regular lock back on the final shot.
I looked back at your earlier posts and didn't see any pertaining to loading, so I must assume that if you do load, you say little about it.
I have three options for you:
One - only shoot light to mid weight bullets.
Two - get a 'can' so true sub-sonics with function.
Three - learn to load and work-out your own loading that will fully function with your selected bullet weights.
I like my 300 BlackOut, don't have or want a 'can' and my heavy weight loads are 150 grainers. The 110 V-max are nice too
Hope I helped, a little.