The volume decrease is most likely because the eCPM is falling at your current source. I would suggest considering launching some identical campaigns to see if they yield more volume. You'll then begin to get an understanding exactly what is causing the decline.
Logically, this should work. Taking the converting metrics over to a new source and then running based off data you know that converts. If it doesn't then you need to look at how they're distributing the traffic and inquire about where your ads are going to be displayed. If you have an understanding at your current source then you can compare and see if it's making sense.
Keep us posted and we'll see what we can do.