I got "seek assistance" again on a recent return, and this time had a friendly and rather chatty border agent so tried to ask if he saw any issues with my passport or why he thought it tells me to "seek assistance" most of the time.

He said the most common reason is not looking directly at the camera. I'm pretty sure I was standing still looking straight into it, but now that he mentioned it I'll pay close attention to where I'm looking next time and see if it makes any difference.