Was it feeding fine before the o ring was installed?
My ICS m4 has a high ROF and without a sector delay chip it has the exact same " every other shot" problem. I would say the 13:1 gears you have installed could be enough to cause the issue of the tappet plate not staying back long enough to feed a bb.
That said, ICS nozzles are just that small bit off from others and I have had an SHS nozzle give me feeding issues to the point that I just use stock ICS ones now.
It really could be either, both are cheap easy fixes though.
If it were me I would do both.
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