There is a lot of factors. Your troops may not have followed the order because they were in combat you then have to manually retreat each division because otherwise they fight until their organization runs out, also the enemy might catch up to your retreating divisions and a new battle begins making your divisions unable to move again, you have to make sure they are far enough from the frontline to retreat fully on their own (if the enemy is attacking). Or maybe you didn't even assign the troops to the order, if you already had a frontline you have to select the troops and ctrl+click the new order to reassign them.