Yeah, the problem is that the bulkhead in question is using a washer as the sealing surface and there is no locknut. The reason a locknut works is because it is a large flat sealing surface that can also be sealed to the nipple with some teflon tape. The washer being loose against a coupling is a major flaw. Throwing another washer and oring at it, for the exterior I assume, does not fix the problem.
The best way to troubleshoot bulkhead leaks is to think like a single H2O molecule and how you might find your way through. I'd go right for the gap between the washer and coupling. Of course it's no problem, that gap has to be at least 1 mil. Once you get past that, you encounter the threads of the nipple. The oring, which is being propped up on the peaks (OD) of the threads is no match. The only way it could work is if it's sitting on the flat of the nipple, assuming it's not a full threaded close nipple.