It is most likely that the types of sensors that this happens on are passive infrared detectors. It is quite possible that your wife’s body heat is outside of the range that the sensors can pick up fairly often.
There are many other types of active and passive detectors but the most common among doors, faucets, and dispensers is the passive infrared detector because it uses very little energy. The detectors are calibrated to trigger when enough infrared light falls on the sensor. If she is small, has slightly less body heat than average, or simply approaches the sensors in a way that does not allow the sensor to pick up enough infrared light then the sensor does not activate.
I’ve noticed on several occasions that I could not activate a specific sensor at my local Wal-Mart. When the door was replaced with similar, but newer equipment, the sensor no longer misses me. I generally have a much lower than average body heat so it was most likely that the old sensor did not have enough range to consistently pick up my body heat. Usually cheaper sensors have much less range and reliability (as is found in faucet sensors where multiple waves are sometimes required to get flow going).
I would be very surprised if she did not consistently get active microwave sensor doors to open for her though. I’d have to say she must be stealth if that was the case. It’s harder to find the microwave sensors anymore, though they used to be used quite a lot on supermarket entrances. The passive infrared sensors are just much cheaper to buy and keep running than active methods even at the expense of not functioning as reliably.