I’m observing 60% fill rate in moPub in one app, 90% in second. What might be wrong? The revenue seems normal (no drop in revenue because of it). Is moPub counting the impressions wrongly? I don’t really have much screen switching that could contribute to it (I move ads from top to bottom in that app, but that shouldn’t reload them). Or maybe moPub doesn’t count its own Marketplace to fillrate?
Do the apps have different geographical traffic?
Maybe one app’s traffic is generating better traffic which advertisers are bidding highly for, while the other isn’t so much.
There are so many factors that can affect fill rate as I’m sure you know, but yeah, it could also be a reporting issue on their end.
Well, the fillrate should be 100% no matter what because I use admob as a backfill (which has 99% fill rate). I have almost 0 clicks reported in moPub for MMedia though (even though there is a lot of clicks in MMedia dashboard of course), so maybe it’s related? 
I’d say it’s a reporting issue then. I don’t use MoPub, but I do use Admob’s mediation. The impression data on there is always more or less accurate, but the clicks and eCPM for traffic other than Admob’s ads are always wrong. The actual network’s dashboards are all reporting correctly though. My advice is to only monitor data per each network’s dashboard, as opposed to the mediation dashboard. 