Hey Richard,
p14s are more sensitive than most of the other species and always show signs of discomfort first.
If the rays are shifting back and forward as if they are scratching the disk there are a number of things that could cause this but all boil down to waterquality. If there is a very fine food residu on the bottom, this irritates. Sometimes if there is food rotting in the filter you cannot measure anything but the water is still bad. Too much ammonia, nitrite, nitrate can cause it. When rays weaken they are prone to parasites that may be there, just only in very small numbers. These start to multiply.
It seems most likely if they are coughing is that they have gill parasites. When rays are attacked by these in great numbers they stop feeding. I would recommend using Tremazol if this is the case.
I cannot see from here so your judgement is most important. If you need any info on how to treat let me know.
Greetings Mark