What evidence tells you to fix conversion before buying more traffic?

If you're deciding where the next chunk of budget goes, a low conversion rate doesn't tell you enough on its own.

I'd want to separate a few possibilities before choosing between more traffic and work on the product or website. Are you reaching people with a reason to buy? Can they understand the offer? Can they complete the next step? And does that step lead to a customer worth acquiring?

My starting point would be to compare new and returning visitors, mobile and desktop, and acquisition sources, then follow the journey through to payment or actual product use. I'd also check that the tracking works before trusting the pattern.

A broken checkout gives you something concrete to fix. A small number of visits gives you much less certainty. Neither is resolved by comparing your overall conversion rate with somebody else's benchmark.

For those who've faced this decision: what evidence made you choose to fix the journey before increasing acquisition spend? And did the result support that decision?