FixedUpdate is governed by the physics clock, making its execution independent of frame rates. If FixedUpdate fails to trigger, the physics simulation is likely paused or your GameObject lifecycle is interrupted.

1. Verify your script is attached to an active GameObject and the component is enabled.
2. Check if Time.timeScale is set to 0 in your script. If this is 0, FixedUpdate will stop.
3. Open Project Settings > Time and ensure the Fixed Timestep value is not zero or extremely high.
4. Confirm that Physics.autoSimulation is enabled, otherwise, physics steps must be manually invoked via Physics.Simulate calls.
5. Look for exceptions in the Console that might terminate FixedUpdate before its logic executes.

Additional Tips

• Unlike Update, FixedUpdate can run multiple times in a single frame if the physics clock needs to catch up to real-time.
• Use Debug.Log at the entry point of FixedUpdate to rule out logic branches like if statements returning early.
• Avoid using Time.deltaTime inside FixedUpdate; while Unity 2021.x+ automatically maps it to Time.fixedDeltaTime, using the latter explicitly ensures consistency.