I,m glad that you asked this question, because you bring up a subject that I wanted raised.
For the past few months my adsense revenue seems to have went "down the toilet", and this month I have not earned even one penny from it. This is despite the fact that I am getting more traffic to my hubs. I only need about £2.50 to reach the £60 payment threshold, but at this rate, I will never reach it.
The culprit may be the HubPages ads. They seem to be going all right, but they really killed of adsense as a source of revenue on HubPages.