Well, it's hard to say, in my humble opinion.
It certainly doesn't sound like you have done anything egregious.
Maybe it isn't so much a filter as much as the whole internal page rank flow shifted with your blog restructuring?
But if the blog still LINKS the same way to your home page / main pages, then that wouldn't really explain it either.
I hope this helps but I don't know if I have helped you get any closer to the truth or not.