From the Washington Post:
The immediate cause of the legal logjam keeping him behind bars is Virginia’s noxious 21-day rule, which prohibits trial judges from reviewing newly discovered evidence if it is presented more than three weeks after sentencing. Exceptions are made only in the case of biological evidence such as DNA.
This month, the judge who sentenced Mr. Montgomery to prison in 2008 was presented with new evidence that his accuser, Elizabeth Paige Coast, had admitted lying about her accusation. Appalled, Judge Randolph T. West ordered the sentence vacated and the inmate freed.
But the evidence came to light only in recent weeks, when Ms. Coast came clean about her deception. New evidence or not, the 21-day rule applies. So Judge West was acting beyond his authority, as the state attorney general’s office ruled, and his order was voided.
The government had to make the exception for DNA evidence so it wouldn’t appear completely mad.