Why do my Meta Ads only work for the first 2 days?

I'm facing the same issue in almost every Meta Ads campaign.

- Day 1–2: Good traffic, good audience, sales start coming.

- Day 3 onwards: Sales drop or completely stop, and the audience quality becomes much worse.

I don't make any major changes to the campaign, but this keeps happening every time.

Has anyone experienced this? What's the real reason behind it, and how did you fix it?

I'd really appreciate advice from experienced Meta Ads experts.

Thanks fro your attention!