Message being received regarding late FPS submissions

SUGGESTED

Every time I process my FPS, I get a message "The FPS that you're submitting for this pay period is being sent after the payment date (process date) for 1 of your employees ; HMRC need you to give a reason why it's late" : But it's being sent on or before the payment date for all my employees. I get this message every week and have checked through all my employees carefully and none of them have their payment dates after the FPS date!! So what can be causing this? Is there any way of finding out which employee it's referring to?