Adsense revenue has stopped increasing.

[kyllua]

My website, which has around one year of history and receives 150-250 daily visitors, has active advertisements. There are also 2 sub-sites attached to the main site, which also have active ads. Everything seems fine, but for approximately 1-1.5 months, the ad revenue, although still visible, has remained stagnant. It is not increasing. There doesn't appear to be any issue with Adsense. What could be causing this lack of growth in ad revenue?