It isn’t something new. Publishers tend to see a drop in ad fill rates every now and then. Sometimes it might be because of the reasons that aren’t in your control. For instance, an economic crisis, seasonality, etc.
But when there’s no impact on the industry and you are experiencing lower fill rates for a while, you should figure out the reasons. Here’s a couple of places to start.
Check your implementation completely. From header bidding to Ad Manager, go through the setup once again to see if you’ve missed to update anything. For instance, Google recently blocked all the price priority line items targeting children by default. Publishers have to update them to resume the line items.
Lack of demand:
Obviously, demand partners are responsible for the ads and if you aren’t getting bids from a partner, then you need to resolve it ASAP. Monitor bid rate and win rate of bidder partners and act as soon as there’s an anomaly.
Some ad blockers block ad networks instead of ad calls. Thus allowing an ad request but preventing ad appearance. At the end of the day, you would see lower fill rates even though you’ve optimized everything.
We have discussed more on fill rates here.