I've noticed that emissaries on fire at will, given an attack move command, will often get stuck in the middle of nowhere. There are no obscacles. The land is completely flat. The rest of the units they are with move forward no problem, but the emissaries keep trying to target things way out of their range and just don't move.
To untuck them I have to give them a move command, then when they start moving, issue another attack move.