You tell your boss,
I am trying to solve a problem, and I can't, try as I might. I've
tried every approach I can think of (debugger, Google, forums, etc.)
and still haven't been able to solve it. Perhaps the bug randomly
disappears on each run and there's no possibility of the bug being
related to my use of concurrency techniques. I conclude, then, that
the bug only manifests under certain runtime conditions which I am
unable to reproduce or isolate.
That is to say: tell your boss the truth. Personally I don't think anyone should spend more than a day stuck on something without talking to other people in their team (either boss or peers) and giving them the opportunity to provide a useful insight or suggestion. Ideally much less time than that, and I think some people would set a much smaller hard limit (indeed one principle of pair programming is that you should spend zero time stuck on something on your own, ever).
Of this, "there's no possibility of the bug being related to my use of concurrency techniques" is a bold claim, of which the boss (or a more senior tech assigned to deal with this problem that you can't deal with) may require substantial proof. Therefore it's worth assembling this proof in advance, because you've quite possibly reached the conclusion as a result of several disparate investigations. You don't want it to come across as either "I have a hunch that it's not concurrency", or "I have rejected out of hand the possibility it's my use of concurrency, because I am so excellent that I never make mistakes". Other than the boldness of this claim, I don't think anything in what you say is particularly out of the ordinary or should give cause for concern.
Your boss is then responsible for assigning additional time from people better able to understand or fix the problem. As others have said, the extra person doesn't even necessarily need to be any better than you, simply a fresh look can help. Your boss or the team might also decide that since the issue is intermittent, what's needed is more precise logging (or other instrumentation) on a wider variety of environments and data, in the hope of catching the bug occurring more times than you'd be able to alone with your dev environment. Worst case, if the bug lies not in your company's code but in some dependency, you need to prove that and you need to give the third party the best possible chance of reproducing it so that they can fix it and provide an update.
If you don't have a boss then a client would be different. The details of what you've tried are no use to them. If you're self-employed contracting for a client, and you have no boss, and you can't complete the project you've contracted for, then in most cases you can't just ask them to help you fix the problem. You need to tell them there's a delay. But you're going to have to fix or work around the problem yourself, or find someone else who can.