What’s the biggest reason your ads fail even when targeting is right?

I’ve seen people blame the budget, the algorithm, and the audience.
But in many cases, the real problem is the creative itself.
Weak hook.
Confusing message.
Poor CTA.
Or the landing page doesn’t match the ad.
I’m curious—what’s the biggest mistake that’s cost you the most money on paid ads?
I’m asking because I’m building Zolvaine, a tool that gives AI feedback on ad creatives before they go live, and I want to make sure it solves real problems instead of adding more AI noise.
Would love to hear your experience.