It's possible for the calibration to be off I guess. Or for the reading of the odd pitch to just not register.
But a lot of the "glitches" that were being reported were not really glitches, just mismatches with pitches that traditionally don't or do get called strikes. Like a big curveball that clips the bottom of the zone but hits the dirt - umps never call it a strike but the robot does and hitters hate it.