Keep riding. Momentary disengagement of virtual inertia will likely resolve itself in a few seconds.
Spin the wheel faster. The wheel must start going at least 27km/hr (16 mph) for virtual inertia to engage.
Make sure you have installed one (1) spoke index.
Making sure you don't have more than one object on your spokes in the path of the optical sensor. A spoke magnet, reflector, tape, or other object could act as a second speed index, confusing the trainer's wheel model.
Try plugging Fliiiight in to the charger. When plugged in, Fliiiight can slightly increase the strength of virtual inertia to optimize road feel. When unplugged, it slightly decreases the strength of virtual inertia to extend your wireless riding time.