voice to text/dictation stops after some time/number of words. why?

Working on transcribing a couple hundred pages of handwritten text by reading it into dictation. It works pretty well but periodically stops without warning or notification. I would be OK if it exited with an alert but it doesn't. The cursor is still the red microphone but nothing is happening. I am using AirPods (Pro 2) if that makes a difference. I will restart/reset them before my next session. My aim is about 2000 words per session so I would like this to work with fewer interruptions.

Any ideas?