Google stopped displaying all my pages in the site: search

[Shirr1]

When I search site:www.mydomain.com I get 1-7 of 310 pages. Then I click repeat the search with the omitted results included and get 1-10 of around 600 pages.

This is only a recent thing. A year ago this would never have been an issue. Im just wondering if others have had this problem and what I could possibly do to fix it.

All my pages are html and content driven so its not as if im spamming google.

anyone got the same issue or any ideas on how to fix it?