A Google Groups thread has confirmed reports from Jeremy Hylton of Google, that Google blog search, in some cases, has issues finding and returning new blog posts in Google Blog Search.
Jeremy explained that earlier on, there was an issue with Google crawling certain blogs. But then at the end of December they fixed that issue. Now, it appears there is an issue with the same type of blogs, where Google is missing random new blog posts from those blogs.
Here is how Jeremy put it:
We are having some problems with your blog that we're still debugging. We have most of your recent posts in the index. At the moment, it looks like the two most recent ones are not getting returned. There was an earlier problem that caused us to stop crawling your blog entirely. We fixed that just before Christmas. I was just checking on your blog to confirm that the fix is working when I noticed that we're again missing a few posts. We'll definitely look into it.
I believe this all started in early November, when Google changed how they index blogs. In early December they vowed to fix it and then in late December, Christmas time, they thought they fixed the bulk of it. However, there are still issues that Google is aware of, including the issue mentioned above.
Forum discussion at Google Groups.