my impressions are at 500-800 per day but my eCPM is 0 to 0.10 $

I have been using unity ads to make money for almost a year now. I had ups and downs, but my eCPM was never outright 0! That is why after 2 days in a row of eCPM of 0 and 7 days of an eCPM below 10 cents I have started to realize that something might be wrong (it also might not and I'm just not understanding something).
Can someone pls help me out to what might be causing my low eCPM? (btw the eCPM 14 days ago was over 1 $))