This is usually caused due to an error in transmission caused by a network outage, or the HMRC gateway experiencing issues. To resolve this issue, do the following:
Whilst in PAYE ONLINE find the FPS in question; this FPS should have a yellow triangle with an exclamation mark in it, which will be in the date column.
Click on the yellow triangle, this should reset the form - which is represented by a green tick.
Click on Send in the bottom right corner.
You should now find that your form has been sent successfully. If it still has not sent successfully:
You will need to keep trying using the above steps at regular intervals.
It may be advisable to contact your IT to confirm there are no network issues.
Also, contact HMRC to confirm there are no issues on their side.