What causes ROAS to decrease?

There can be many reasons why your ROAS can decrease, especially on Facebook. You haven't been side-swiped. That's not how Facebook ads work. Here are the top 2 things that can reduce your ROAS:

1. You have fallen prey to target audience exhaustion. Your ad has been shown to most of the audience on Facebook. Consider increasing your warm audience circle.
2. Your budget might be too low.
3. Your cost per click has been enhanced.
4. Your average order value is decreased.