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How often do you do yours?
Although i do lots of updates on a daily basis i dont add pages that often, would this be the reason Google hasnt crawled my site since 22nd August as its starting to worry me now...?
If you have old pages and you feel Google has not updated the information indexed for your site, you might want to check making correct entries in the last modified and and the frequency you want Googlebot to revisit your sitemap.
Now although this is what these XML tags were intended for, I really do not pay much attention to it. I prefer getting a new fresh forum or blog link instead because I feel tired editing the sitemap.xml file if it is not powered by some CMS for seldomly updated sites.