Hello. I want to use rigidbody.AddForce, however I want it to be unaffected by gravity and always have the same jump force.
If you jump without gravity then you’ll continue moving up and never come down. 
Gravity is always consistent therefore it must be the force you apply or how many times per second you apply it that changes. Perhaps you are adding a force in the ‘Update’ callback (per-frame) rather than ‘FixedUpdate’ callback.