Hotblogs has a threshold of 14 days, it only displays the most recent (14 days is default) most visited blogs. So if these Blogs are not hit for 14 days they will no longer show up in the Hot Blogs.
This default can be changed in roller.properties. The default is:
homepage.hotBlogSince = 14
Cool.. thanks for this. I will make the changes on Yellowbox so content stays up longer.