Resistance on hills-Tacx Flux

Yesterday, I did the Zwift Fondo 79K. (I registered for the 53K but somehow ended up in the longer ride. OUCH)

The first time up EPIC KOM, no resistance happened, so I was going up it at 145W the whole way. It was working fine on other hills and has worked fine pretty much on all my rides. I have been using Zwift since day one. I have an extension to the ANT+ dongle right above my trainer.
The last EPIC KOM the resistance was there, and I was DYING. So, how come it does not work the first time and it does the second time, IN THE SAME RIDE? I use the Garmin Vector 2 Power Pedals for Power and the Tacx as controllable. Not using anything HR or anything else. Thanks