Welcome to the forums @speedkilz420 
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@GamecockOperator may have some information about this based on his experience.
What magazines are yall (who are having trouble) using ?
I put subs and supers through mine today no issues. @GamecockOperator had trouble with Daniel Defense mags. I used Lancer and Hera issue free. Both of us used lithium grease on the carriers and lubed good. My .300blk with supers and a can (gas position 5 was best for me) was stupid quiet.
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I think i got mine worked out. Put 160 round of 110 and 168 grain through no issues.
I polished the bottom of the bolt carrier a little dremeling on the pointy side that was milled crooked. Made nice and smooth. Look up a gun tuber channel named 2a cop. On youtube. He has a video showing how its machined a little crooked just like mine.
Not saying do this. Its wiser to send it in and wait months. Hopefully this slight mod doesnt void my warranty. But it worked for me so far. Havnt tried subs yet
Thanks for this post. I had switched to a Noveske KX5 to get more backpressure, and mine still didn’t cycle. Went through and greased everything and got some Lancer 300 BLK mags, and now it runs perfectly. I had been using Pmags.
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Glad you’ve found some success and my experimentation was found useful! Happy shooting! 
Welcome to the forums @Cipher
That is Hornady Sub-X ammo. That is what I’ve been running in my Jakl and I’ve had no problems with it at all.
Back to having issues with my 300 BO JAKL upper. Issues are failing to feed, ejection is 3 to 4 o’clock but not chambering. Starting to get dejected. Gun is clean and have tried several makes of mags to no improvement. 5.56 JAKL upper runs like a champ with 0 issues with any ammo or mags on same lower. Waiting on my Primal suppressor to get out of ATF jail, does the 300 JAKL need to be suppressed to run, if so i dont see how that will help the failure to feed issue.
Which gas block settings are you using?
Max has setting. Anything less and gun won’t even eject.
I just shot my brand new 5.56, 13.7” JAKL today for the first time. I had tons of feeding problems using a gen 3 PMAG. But the instant I switched to the gen 2 that was sent with the rifle it had zero issues. Not sure why that made a difference but it did
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Welcome to the PSA community @GoosenamedMoose 
My 10.5 556 will cycle pretty much anything I put in a mag. I rung 40 round Pmags, aluminum mags, lancer, 30 round Pmags with and without window it runs flawless with steel and brass. My 556 is not the issue, it’s my 300 blackout that has issues.
That’s was me, need to go back to the range and make it is still good to go
I’ve had this exact same issue with mine on supers, I was told it was my ammo since the subs cycled fine. I sent my ammo back to the maker and still haven’t received my new ammo to try yet. But subs run fine it’s just supers I’ve had problems with. I had to mortar mine on the bench to get one of the spent cases out of the chamber.
Did you change the gas settings between running subs and supers?
Yes and tried the supers on each gas setting. Some actually ejected just didn’t pick up another round and the others I had ejector swipes and the one I had to mortar actually blew the primer out. Which is why I sent the ammo back to the maker, all 700 rounds. This is a custom ammo maker not a big time ammo maker. It’s a SBR load for 110VMax faster burning powder to get all powder burn out of a 6” barrel. Since these were made by him for a different little PDW 5” 300 I had I thought maybe they were just too hot coming out of an 8.5” barrel. He is reloading them to a regular 300 load and sending them back but has been swamped for a few weeks so I haven’t gotten them back yet.
Try some factory ammo…if he pulls the bullet you are dealing with remans, and i have seen many a reman blow up a gun.
I got my 300 BLK 8" JAKL put together and took it to the range. I used Winchester subs and Swiss RUAG Supers. I ran my 762 Flow Ti can for both. I had feeding and ejecting problems with my PMAG Gen3 BLK mags for the first 5 mags. I had to mortar it repeatedly to get rounds out. The bolt wouldn’t go all the way home, I would get a light primer strike,and I had to mortar it out. I tried all combinations of ammo and gas settings.
I let the gun sit for 15 minutes, thinking about what I could do differently. When I went back it didn’t malfunction again for the last 3 mags. I’m stumped 
. We’ll see how it goes on my next range trip before I make a warranty claim.