I suppose the first question will be why you need an extension. If there were truly extenuating circumstances (e.g., you've been out of the country for six months with no access to an appropriate facility in which to take the exam) . . . well, although a good excuse, they might ask why you are just bringing this forward now.
If you had a medical issue, you still should have taken the exam; there would have been a remedial until the issue was resolved. I suppose if you've been laid up in a hospital for the last few months or otherwise have been unable to travel to the exam site . . . again, you should have notified folks ahead of time. Simply having a medical issue you wanted to wait to be over is not a good reason for not taking the exam.
If you simply procrastinated . . . it will depend on the accession source whether or not they will let you take it late. I don't believe the March 1 date to have started the process is a legal requirement, so it could probably be waived. But I would be prepared to explain why you need the extension.
And, you should contact your accession source (e.g., ROTC, USNA, etc.). If there is more than one, lots of calls to make.