Ad fatigue is one of the most common issues when running Facebook and IG ads. It's when an ad or even an offer is shown too much to a particular audience over a period of time where they become tired of seeing it and the click rates start to drop therefore increasing costs.
Follow these steps to spot any issues:
If you want to make decisions based on the big picture- which is always a good idea, change the view to weekly or monthly.
Also if you'd like to spot issues regarding declining clicks or conversions, then customize your Trend Chart Metrics (Y-Axis) to include: Link clicks, CPC (Cost Per Link Click) Leads, Cost per lead, and Purchases or Cost per purchase.