1. It's generally easiest to do BEFORE the judge signs off on it. That wouldn't apply here.
2. It must be done for a good reason, and should be done soon after it was signed. It's almost unheard-of for a judge to allow a guilty plea to be withdrawn more than a year and a half after it was entered.
Because it was induced through both fraud and force. That is sufficient to invalidate any agreement.
Flynn got effective council and withdrew his guilty plea in January. This would have forced the prosecutor to provide evidence in a trial. However the DOJ decided not to pursue the case.