Last months I have noticed a strange behaviour in Google's Webmaster Tools.

Although I provide a valid Sitemap for my website, which is normally processed and without any issues from Google, while the number of Submitted Webpages keep rising (30.923 pages), the Indexed Pages don't rise anymore (26.819 pages since the end of January).

Checking the Crawler Stats, everything seems normal and there are no Crawl Errors.

There was no significant changes to the webpage these last months.

Anyone noticed a similar issue and any possible suggestion?