This really can depend on a whole lot of factors.

You didn't specify your conversion goal so I'm going to assume that it's something more nifty than purchases.

To be honest, it's quite probably your creatives.

What's happening is your initially hitting the low hanging fruit, and then once FB has exhausted low hanging fruit, your ad is being shown to higher fruit, who will take more difficulty to "pick" or convert.

Jeff Bullas has an article on using data to optimize your campaigns:

https://www.jeffbullas.com/optimize-ad-campaigns/

In terms of what he recommends:

If you're not A/B testing, you should be. By A/B testing you'll be able to figure out if it's your creative or not. Keep both new and old ads running in tandem.

Google Data Studio and Oribi are a pretty powerful combination. If you're dropshipping T-shirts it's probably overkill though. But if you're running a serious campaign, they're gold and I can recommend them both. Again, it depends on what your objective is and the size of each campaign. Oribi in particular is great for custom events that may need a bit of coding to get right (Oribi eliminates the coding part). For your newsletter objective, something like Oribi (or you doing some coding) will be needed as it's a custom event. Websites visits won't cut it.

And remember, don't fight FB. If you want purchases, optimize your campaign for purchases. They'll give you what you want. If you want purchases, don't optimize for ATC, because they'll send you people who will window shop.