How do you approach difficult unreproducible bug tickets?

I have a bug ticket that seemed simple at first but is proving to be very difficult. After reaching out to relevant parties I cant reproduce the issue. Theres about 3 services and 2 workers with old convoluted code so it isnt easy to follow the trail.

Ive spent a handful of hours on it so far. What would you do at this point? The issue isn't major and happens infrequently.