Hi James, I assume that Google indexing your sitemap isn't the ultimate goal. Rather, Google indexing your individual web pages and ultimately, your pages showing up in the top of the search results is the ultimate goal, is that correct?
Your sitemap is only a suggestion to google that you have pages you would like it to index. Google may or may not crawl pages just because they're listed on a sitemap. Roughly, its the authority of pages to be indexed that determines frequency of google's crawl--and that even includes the sitemap. The greater the authority the more often they're going to be crawled.
The last time your sitemap was revised, it may have been later in the crawling cycle and this past time, it may have been earlier in the cycle (or you domain could have lost authority). Ultimately, social networking, link building, and building authority is going to be what speeds things up for you. In the mean time, be patient, you're on Google time, now.