Normally plans filed in the US are valid 30 minutes before and 120 minutes after proposed departure times. To check the flight plan call Flight Service (1 800 WX BRIEF) and ask them to call center and check that the flight plan is in the system, but not earlier an hour before the proposed takeoff time. If you go beyond the 120 minutes valid time then re-file dont send a delay message as in my experience, delay messages do not wortk in the US.