at first glance, your sitemap seems do not have problems. some time ago, I had a problem like this with a website of a customer (electric supplier) and by using a little trick, after a reasonable time, we have increased the numbers of pages (among those included in the sitemap) crawled by google. If you want, I will do a modification to your sitemap generator (I think that your system already have a sitemap generator) in order to try if this method could help you too.