There are a few common fixes depending on the exact issue, so the easiest troubleshooting method is to just try each one to see what helps.
Make sure your Sample Rate (App CFG → IMU) is set appropriately for your controller / IMU. The Little FOCer V3.1 uses the LSM6DS3 IMU, which we recommend setting to 832Hz (though feel free to try 416Hz as well if you’re having issues with 832Hz). Additionally, Loop Hertz (Float CFG → Specs) must match or be a multiple of this Sample Rate. If you are running a lower Sample Rate such as 416Hz, it should be doubled for Loop Hertz (832Hz) for better performance.
Experiment with different values for Zero Vector Frequency (Motor CFG → FOC → Advanced) in the range of 15 - 30kHz (25-30kHz is most commonly used). Just try one value at a time in increments of 1kHz or so to see what sounds and works best.
WARNING: Going above 30kHz poses risk of bricking your controller. It is not recommended to do so.
If neither of these work, make sure your Gyroscope is properly calibrated. Navigate to IMU Setup (Start tab on Mobile, Welcome & Wizards tab on Desktop) and go to the Calibrate Gyro step. Make sure your board is still (orientation does not matter), and wait for all axis values to stabilize. Once they stabilize within +/- 0.02 or so, save and exit the IMU Wizard.
The above is assuming you have an actual working Motor configuration, but if all else fails, re-run Motor Setup to check if this is where the issue lies. Be sure to change whatever usual values necessary after running Motor Setup, such as currents, voltage cutoffs, and observer gain.