Sorry to know but the general view about advertisement networks is that they pay lesser and lesser everyday, you may account that to growing competition in your niche or the loss of demand for whatever niche you are targeting for. Google is known to change every now and then, try to push in the best and relevant content, try foir sometime if you still get peanuts for a payout then ditch the niche, there is nothing more that one can do regarding this.
I would say that the problem might be either competition or overall site quality. If google detects that you have duplicate content and you're not an authority site, it won't value your clicks as much. I've making a lot more money from google since two weeks ago. From what I note, google won't also assign a good price tag to your clicks if they don't convert and will assign to other sites with a better historical data for click through rate.
Nope, no changes. My niche is only get more popular as time goes on, so it's not an issue with popularity. It maybe duplicate content.