The URLs not accessible error in Google webmaster tools, is often just a temporary problem. But you should take care to check the URLs reported, to make sure that the problem is solved. If there is a real problem, then its likely because you recently changed something on your server, or in one of your scripts.
Also make sure that your sitemaps will take advantage of caching, this can be done by Using HTTP IF MODIFIED SINCE with PHP.
When we tested a sample of the URLs from your Sitemap, we found that some URLs were not accessible to Googlebot due to an HTTP status error. All accessible URLs will still be submitted.
The HTTP status error could indicate a 404, so make sure that database queries are completing as intended, and/or that the pages exist on your server and haven't been moved accidentally.