Can anyone explain this electrical/generator mystery for me?

Last week the power line maintenance company shut down power for 6 hours, because I have need of medical equipment which runs on electricity I hired an inverter generator. The medical equipment normally runs through a UPS backup battery which gives me about 4 hours so I simply plugged the UPS into the generator. Everything went according to plan until I discovered that the UPS unit was discharging itself despite being plugged in to the generator, I needed to then plug the equipment directly into the generator. Can anyone explain to me why the UPS would discharge fully despite it being plugged in to the generator. As soon as the electricity came back on and I plugged the UPS into the mains it began recharging. To deepen the mystery further, earlier in the year I hired a different generator which I used for about 5 hours, in that instance the UPS did not discharge and retained full battery charge. If anyone can explain to me why this happened I would be very pleased, thank you.