Andy_Langton - 12:29 am on Mar 5, 2013 (gmt 0)
Based on your description, I'd say you are seeing Google's raised spam thresholds. I would assume they do this in response to the short shelf life of your common or garden spam site.
Google seems to apply particular filters to homepages (given their inherently stronger ranking strength and thus ability to target more competitive keywords).
But you should be looking at days or weeks tops, not months. So, you're perceived as borderline. The fact that some make it through and others don't means that IMO you need to raise that borderline pretty quickly :)
If you launch with keywords above certain degrees of competition in your titles, and/or tick all of the SEO boxes you're also placing yourself in a higher degree of scrutiny. Disappointingly, I've found the quick fix with Google is to throw links at the problem - or be prepared to wait it out.