How come when you go to a page and see google ads, and you keep reloading, sometimes you see ads, and sometimes it is blank? And this site isnt necesarilly a small site. It could be huge, and high in PR.
It is usually because you are refreshing the page. A lot of factors play in this including cookies, cache, and refering URL. Try navigating around the site through links and you should see ads nearly all the time. If you don't there just isn't enough content on that page to bring up an ad.
All my sites hosted on high speed servers get the pages showed long before the ads does. As Riboflavin said before, many factors are involved into the time formulae. As a side, that's why fast page rendering and loading is crucial for all bots, including the AS one (G MediaPartners).
How much content is needed to bring up an ad a good percentage of the time? I have an entire article, but it may not be enough? here is one of my pages on which the ad spaces are often blank. http://www.begin-weight-loss.com/WeightLoss101/WeightLossstatistics.html
When clicking through from http://www.begin-weight-loss.com/Weightloss101.html I see 9 ads, which is a lot. But when going directly I see 6ish. This is simple because google thinks the keywords in that article are most relevant to those 6 ads. Any other ads that could be displayed would not be relevant. If you are really gung ho and having 9-10 ads display all the time on everypage (which doesn't necisarily mean more $ ) then you could try adding more content to the article, but keep in mind it will take a while for the adsense bot to be back again.
Ok, that makes sense. Now what about the PSA ads that I often get on that link i gave you as well as the other article pages. Sometimes, it gives me PSAs. I though you only get those when the robot doesnt know what kind of ads to give you. I realise you were saying there may only be 6 ads that the bot thinks are relevant, but sometime I will get 1 PSA and 2 blanks.
So how about this. Ill set white as an alternate color for the top 468x60 ad. For the top 336x280, Ill have an alternate ad which I created. And use that collapseable script for the bottom one.
With that many adds, I would definitely recommend using some channels and figuring out which ads are the top performers. If there are some that stand out you might consider removing the others so that you don't wind up with PSA's (or white space) in your "prime real estate".
Sounds like you're on the right track then. When I looked at your page I saw about 10 ads in 3 blocks, so it must have been picking it up that time. I've noticed ads on my sites go to PSA occasionally too but it seems to happen very infrequently now that they've been up for a while.