Use a higher gear. Fliiiight has a square power curve, so doubling your wheel speed will quadruple your watts. Bonus: Gearing up also improves road feel. Note: You should always use your fastest gear in ERG mode.
If your trainer stops changing resistance, troubleshoot your connectivity over Bluetooth or ANT+. When Fliiiight stops receiving commands due to interruptions in connectivity, it will keep implementing the last command it received until it reconnects. The power reading may also stop updating, or drop to zero at the same time.
If Fliiiight never seems to engage the magnets after the initial start-up sequence, make sure you have one (1) spoke index installed on the wheel, near the wheel rim. No spoke index, more than one spoke index, or additional objects such as a spoke magnet in the path the the speed sensor can interfere with Fliiiight's wheel model. Continue troubleshooting Fliiiight's wheel model.
Open the Chrome troubleshooting app and set the motion limit in a couple steps at a time. Note that while adjusting this setting can increase Fliiiight's maximum resistance, power, and slope, and increase the strength of virtual inertia, it will also increase the likelihood of wheel-magnet contact. How far you are able to customize this setting depends on your individual wheel.
Fliiiight can represent up to a 7% grade in SIM mode. This means you might not gear down as much to get up a steep hill, but it will require the same total power. This affects SIM mode only.
Check that the trainer has not released the magnets and shut down due to low batteries.