Google cache

site is being cached very infrequently

[mcfc28]

Sorry If this has been posted before.

I have a page rank 5 site, which is getting cached less and less frequently. The main homepage is only cached every couple of weeks, while other pages with PR4 are being left upto for 6 weeks.

Most pages are updated at least every week and I have submitted a sitemap. The webmaster tools, will not allow me to increase the crawl rate either.

Does anyone have any suggestions?

Thanks
Chris