new 300 whisper cycling problems

[GaryM]

I just finished building my first whisper on an AR15 platform. I range tested it and ran into this problem.
It chambered and fired great, I see a lot of potential accuracy in it too. it would eject the fired brass properly too but it wouldn't chamber the next round. I am 100% sure it isn't running the bolt back far enough.
I remember seeing a fix for this but since I can't seem to find it again I wanted to make sure. I believe it was simply swapping the rifle type buffer and spring for a carbine buffer.. or maybe it was a carbine buffer and spring. Can anyone tell me which it was or maybe an even better fix?
Right now I have a .125 gas port with a standard gas tube. (An adjustable gas tube is on order but won't be here for a few weeks)
The ammo I was testing was
1. 150gr fmj/8grs wc680 (fired, ejected, wouldn't reset trigger)(should be subsonic)
2. 150gr fmj/9grs wc680 (fired, ejected, wouldn't reset trigger)
3. 150gr fmj/10grs wc680 (fired, ejected, DID reset trigger)(supersonic)
Thanks in advance!