Search debug.log for "proof-of-work found". If you find any, then check for any errors right after that.
Quote from: davidonpda on August 19, 2010, 07:43:01 PM (https://bitcointalk.org/index.php?topic=862.msg10291#msg10291)
How big of a margin on the time is allowed for things to work right.
The margin is 2 hours.
This should be solved in SVN rev 141 and the next release (0.3.11+). It'll pop up a message box alerting you if your clock is off by more than an hour.