Google occasionally slows down the crawl process. Generally, you shouldn't worry but I suggest you to keep an eye on your Google Webmasters Tools account.

I remember and interesting article about Google and Last Download Date. Thanks to delicious, I found it! http://www.seroundtable.com/archives/019843.html

Here's an interesting quote.

A Sitemap file is great but it does not mean Google has not indexed your content. The Sitemap file is one way that Google may find your content, but Google typically finds your content other ways, and typically Google may find your content faster without the Sitemap file. Meaning, Google will typically find new content on your site through their normal crawl process, faster then they would when compared to the Sitemap file. That doesn't mean that the Sitemap file doesn't help, I believe it does help.