I was going to create a similar thread but I'll jump in here - similar question and I'd love the advice of members.
I also built an Aero EPC 9mm Glock lower out with a carbine length tube, PSA 9mm bolt (14.18 oz) + FM 9mm buffer (5.47 oz, does not appear to be "longer") for a total weight of 19.61 oz. Odin Works barrel. Flat-wire carbine spring. LaRue MBT-2S trigger. Well lubed. Lots of FTF. Tried swapping out with my FM9 bolt (nearly the same weight 14.14 oz). Same problem. Cannot get it to feed consistently - even had one case of brass getting into the FCG.
Met up with a guy at the range running a similar put-together PCC with the Gentle Recoil setup: https://blowback9.wordpress.com/2023/05/28/9mm-ar-gentle-recoil-buffer-system-final/ - he had a KE lower (so no flat wire spring) and it was night and day difference in feel... and it ran without a hiccup.
Seems like it all has to do with weight, and I've seen similar advice to this all over the internet: Buffer system+bolt mass should always total 22oz or more. If that's the case, why are most common off the shelf pieced-together setups coming in under 20 oz?
I've got the components (JRC longer tube, weights, heaview buffer) coming in to put together this system and see how it runs... but I'd love to hear from the experts here around tuning these beasts.