PPC Keeps Humbling Me

I’ve invested a lot into ramping up my PPC ads, and although I was operating at a loss initially, I was able to collect a lot of data. Afterward, I started refining my ads, following the usual strategy: moving from Auto to Phrase, then to Exact. My conversion costs were coming down, but suddenly, my sales dropped significantly, and now I’m back to being upside down with my ad spend.

What could’ve happened?