Google is very lazy indexing pages of our site (taking more than a month now). It has a sitemap that is valid, it has an html version of the sitemap, there is nothing wrong with the internal linking structure and Google has downloaded our sitemap and crawled our index page on several occasions now, but fail to index new pages and update existing ones that have been updated with new content.

On each page there is a dynamic banner on the left that displays one of our top 3 performing products each time a page is loaded or refreshed. I have done this with PHP and not Javascript, so search engines may get different code in this part of the page each time they crawl our site. It's only a little piece of code, an <a href> statement containing a different url each time and an <img> statement containing a reference to a different image file each time.

Could this perhaps make the page too 'dynamic' to Google's liking and that's why they are taking so long to index/update the page?