How do you diagnose a flow that looks healthy but may still be missing eligible customers?

I'm curious how other Klaviyo users diagnose this situation:

A flow appears to be running normally and generating revenue, but you suspect some customers who should have entered the flow never did.

What do you check first?

For example:

- trigger/event data
- profile activity
- flow filters
- suppression/consent
- timing
- attribution

I'm particularly interested in real Shopify + Klaviyo cases where the problem wasn't obvious from the dashboard.

If you've dealt with something like this recently, I'd love to hear what the first sign was.

Happy to discuss by message if you don't want to share store details publicly.