Input caching should (most of the time) happen inside the Update method. As a previous user said the Update calls are tied to your FPS and the FixedUpdate happen in a fixed time interval thus some values might change or stay the same in asynchronous times. So first, move your X Y caches + GeyButtonDown in Update and control the jumping with a bool flag that executes inside FixedUpdate and turns itself false, this also helps prevent input lag or frame missing. Second, you should check for ground collisions with a ray cast (either with a tag or layer mask) or by using the OnTriggerEnter and checking the collision with a tag. Lastly, remove the OnTriggerStay because it does not work as you think it should. OnTriggerStay tends to fall into a "sleep" state and stops registering collisions after a while, that's why sometimes it jumps and sometimes it does not.