I just figured out what was causing an intermittent problem with my Mini 9's built-in trackpad. Occasionally I'd find my trackpad not responding reliably to two-finger gestures. Firm or light touch, I couldn't get it to move more than very short distances. After an hour, it'd be fine again.
Turns out it was because my hands were too clean! If I'd just gone to the bathroom and used some liquid soap to wash my hands afterwards, the trackpad couldn't reliably sense my touches. Over time, the remaining residue would wear off or evaporate and it would work reliably again.