Recently, something I was working on had a bug. Only in specific cases. Not particularly large. etc.
I didn't immediately find the source of the bug, and, more importantly, validating a find took an approximately overnight run.
In the meantime, between long runs, I worked on a ton of other little improvements. Refactoring. Adding performance optimizations. etc.
And then just as I was nearing running out of such things to do, I looked at the logs of one case I was optimizing and saw instantly the cause of the looming bug.
Just keep working on the system until you understand it so thoroughly and so intimately that you sweep up all the bugs as you go.