Andy,
Looking at some of the posts around this issue, most of the sites in question are low ranking sites or sites that have been blocked for some penalty. I haven't looked at any of your sites and if you can share the URLs I will be glad to review them or have Brett review them for you.
Time scale has nothing to do with why sites "drop off". Sites rank changes as we review the factors that make up the ranking. You should use the Webmaster Center to determine if your site is being penalized.
When you first launch a site it can take some time to build the link equity. My own personal experience with this was that I was crawled within a few days of launch, but didn't begin to rank for my keywords until a couple of months and lots of link building.
You should read this...
http://blogs.msdn.com/webmaster/archive/2008/01/11/i-m-not-ranking-in-live-search-what-can-i-do.aspx
It is important that you review the details we give in that post. There is no magic or mystery if you follow those recommendations you should see crawling and ranking for your sites, but if you are doing shady link building or made for adSense, then you may not get your wish.
Cheers
Jeremiah
