Have you patched to the latest version? In old versions, you can hit the unit limit for the game which will stop new units from being generated, whether it is summoned or recruited.
Are you sending horrors against the PD? Maybe he just stopped putting any PD on the provinces that you targetted.
There was PD in the province because I sent a scout there that turn to claim the province that the horror was to attack. And I havent reached the unit limit because I managed to summon some units after the send horror spell was cast