Many of the things that happen have nothing to do with AdSense whatsoever, but have more to do with changes in traffic and advertisers.
For instance, my weekends didn't used to slump quite as drastically yet now FRI-SUN isn't very good yet my traffic stays pretty constant. Turns out in this economy some advertisers pull their budgets on the weekend so MY best money is MON-THUR. Other sites do better on the weekends, it all depends on your niches.
If you see a serious slump what should you do?
- Check the ads on your site from multiple geographical locations (use a proxy) and if you see some real junk, dump 'em in the filter. Every now and then some big ad campaign will clog up bunches of valuable ad space with stuff not terribly relevant to your visitors and it needs to be stopped ASAP.
- Check your traffic patterns and see if you had a change in keywords bringing traffic. Often you'll see a term that used to bring traffic that slipped down the ranks that needs some SEO work to correct. Sometimes new traffic from a different term or new source is less relevant and can theoretically drive down your income and may need to be blocked.
- Disable some low paying pages for a period of time so AdSense will 'forget' about them. I had a bunch of pages get caught in some AdSense trap that changed the quantity and quality of the ads on those pages, including spitting out PSA's. Made some changes and it didn't seem to help when it should've, so I took AdSense off for a while and restored it a month later to massively better ads and payouts.
- Check your SEO. For instance, a few AdSense people I've helped out of a rut in the past had seriously bad SEO on their sites and fixing that was all it took to make their site pay 3x more than it currently was. AdSense is quite literally a window into the soul of Google and the more targeted your ads, the better targeted the SEO and my pages improved in the SERPs. This yeilded very targeted traffic and very targeted ads which resulted in a lot more money.
- Help set the "BUY" mood for the visitors that click through to the advertisers. I'm a firm believer that the ROI of the advertiser is a factor in smart pricing and contributes to the actual payouts. If your site is just trying to get people to click off the page as quickly as possible to collect your PPC fee, then I would expect lower payouts. The more value you provide for the visitor, the more value you provide for the advertiser, then it's more likely the advertiser ROI will be higher when the visitor clicks and your payouts will probably be higher as well.
Lastly, none of the things I just mentioned are a one-time fix.
Typically you have to keep reviewing and updating all the time and adjusting to keep it all on target.
Lots of other factors involved, too many for a quick post, maybe some other day.