Helping troubleshoot an AR9, Glock Style. Getting very little to no primer strikes on every 10th round or so. Rifle Build:

Before he had suppressor, he had failures to eject, but no other issues. With suppressor it is feeding reliably, but he is getting no to extremely light primer strikes sometimes. Other times it is fine. Never on first shot. We can shoot both my and his upper on my lower without a problem. However, both uppers have a problem on his lower. He has a drop-in trigger that keeps both uppers from pinning on because it is not rounded on the top of the hammer and the hammer hits the bottom of the firing pin unless you lock the hammer back. This is what makes me want to replace his trigger with a regular one. But he also has a very special stock assy that is not of regular design, and buffer is super light (but says 9mm on it).

What could be going on here?

Build:
Unknown drop in trigger
QC10 Glock Style Lower
ARC-X 9mm Stock and 9mm Buffer (https://mvbindustries.com/product/arc-stock-9mm-multi-pos/)
X-products side charging upper
QC10 Bolt
QC10 5" barrel
Omega 9K Suppressor

I don't have any experience with suppressors but since both uppers work on your lower I would rule that out. I would experiment with the trigger and hammer, replace with a standard mil-spec trigger and see what happens.
Also the buffer and spring, try a standard carbine buffer and spring to slow down the action.

Make sure you only change one thing at a time to rule out changes.

AR-15/9 is more correct, it's a caliber swap on the basic AR-15 platform, not a new model designation or rifle.

If he didn't have problems before the suppressor, then it's probably an increase in backpressure adding velocity to the bolt when the suppressor is affixed.

Usually remedy is a heavier buffer.

That he has a super light buffer, only means it's more likely than not he needs more buffer.