There are a bunch of write-ups on Hub pages for this one, but the basic reasons are:
1. You do not have 6 months history yet
2. The Hubs may be too short
3. The Hub may have copy write issues
4. Hub may lack interesting content.
5, Info supplied when applying did not match other details you supplied to Google.
From what I have read, once Adsense rejects you the 1st time there is only a 10% chance they will approve later. It is better to start with a new user name.