ar15 help

So I decided to build my own 5.56 Ar15, ordered the parts I needed and after they came in I sat down and put it all together every thing looked good.. at least I thought so went to the range to test it out and it would fire one round eject the round but not reload.. did all the safety checks and reloaded and fired same result repeated several time but got the same result. So I decided to take the BCG out of my other AR and put it in the new build and just like that every thing fired correctly. So I decided to put the new BCG into my old AR to see if it would firer and everything worked as it should .. so I put the BCG back into the the original weapons they came out of and went to firer the new build with the original BCG it had in in and it would firer once eject round but not reload…

Can any one give me any ideas on this seams out there to me any help greatly appreciated

Check the alignment of the gas rings. They should NOt be aligned OR it needs a break in- Coat the exterior of the bcg in some oil and run rounds through the carbine. Cycle manually if you have to. Clean off oil/powder and repeat the process.
You may be dealing with tolerance issues as well as wear allowing this to happen.