Two 9mm builds, both exhibit similar problems; failure to eject reliably (one much worse than another).

#1.  PSA upper & hybrid bolt.  Matrix Aerospace colt pattern lower w/Kak 9mm extended-heavy bolt.

#2.  Colt upper & bolt.  PSA colt pattern lower w/PSA 9mm bolt.

Both platforms extract well but #1 will fail to eject sporadically; usually within the first and possibly second rounds.  After that it usually functions well.  #2 flat out fails to eject on every single round.

Failures manifest as stovepipes or spent shells that do not eject and remain with the bolt.

Any ideas ?  Thanks.

Try bending or adjusting the ejector so it sits closer to the bolt

That's what I had to do for mine and works great now. Get it as close as you can to the bolt.

What for ammo?
Have you tried different ammo?

Bend the ejector inward towards the bolt center line.  I built two PSA dedicated 9mm ARs.  One had sporadic failure to eject where empty case stayed in gun the other had the same problem with 100% failure to eject.  I bent the ejector inward on both and it cured the problem completely.

Did you manually cycle rounds through the action before shooting it?  How do/did they extract/eject?

If the ejector adjustment doesn't solve your issues fully, you can look at the buffer length/weight and or ammo (low powered?) not allowing the bolt to fully cycle.

Check if your bolt is dragging (possibly on bolt catch or elsewhere) and slowing the action.

What mags?  Sporadic issues could be mag related if the mag is dragging the bolt or otherwise interfering with proper ejection.  Are you by chance holding the mag when shooting?  I have seen several AR9s where a shooter holding the magwell/mag has caused intermittent or consistent ejection issues.