Sounds like an undergassing issue. If you used an original Colt 10.3 MK18 bbl this may help. The 10.3 was orginally a request by SOCOM for a CQBR length upper vs the 11.5 which was the standard at the time. The original gas port sizes for the 10.3 were .092 (which was way overgassed) and required the use of a H2 or H3 buffer and an increased weight buffer spring to slow down the bolt carrier and prevent bolt bounce related failures. They then experimented with a .067-.068 gas port and found to get it to run correctly the weapon needed a regular carbine buffer and spring and had to use hotter 5.56 ammo like the m855/ss109 or run full time suppressed with an H buffer. Eventually the guys at Crane found the best combination was a .070 gas port with a H1 buffer to run suppressed. If running unsuppressed, it needed a regular carbine buffer and 5.56 ammo. FN/DD and even Colt are now making the barrels with a .074 gas port, which provides enough gas to cycle with .223 or 5.56 ammo, Using a H buffer both suppressed and unsuppressed. They also added an O ring under the extractor to aid in proper extraction.
If your barrel was cut down from a longer one, then gas port size could be the issue as well since longer barrels use smaller gas ports. The longer barrel allows more dwell time for gas to feed the system.
I guess the solution to your problem lies in figuring out what size your gas port is. If it is the .070 (the correct Crane spec), or .068, you may have to run it with a regular 3.0 oz carbine buffer to get it to run unsuppressed.