Google Calendar currently does not keep calendar feeds up-to-date well. Google is aware of this issue, but they have not announced any action to change it in a couple years. The Google website displays the following message regarding the issue: http://www.google.com/support/calendar/bin/answer.py?hl=en&answer=38847
You can report this issue to Google at: http://www.google.com/support/calendar/bin/static.py?page=known_issues.cs.
We recommend that you instead sync directly with other calendar applications such as Apple iCal, Microsoft Outlook, Mozilla Sunbird (http://www.mozilla.org/projects/calendar/sunbird) or you might also consider looking at Yahoo's online calendar (http://calendar.yahoo.com).