I know you can resubmit as soon as you have a "significant" change to your application (i.e. new ASTB scores, GPA change, etc.). Are you still in school? If so, how were your grades this past semester? If you are out of school or didn't improve your grades, I suggest retaking the ASTB (if you feel you can improve), then resubmit.
I thought 6 months has always been the minimum time to reapply without any improvement. Might be wrong...