AddForce with direction tuning

Hello everybody. I´m using this script for moving my Rigidbody Capsule Character.

 void FixedUpdate () {
     //Only move if on a solid ground
     if (onGround && collisionAngle > slopeAngle) {
         //Get desired velocity
         velDesired = new Vector3( Input.GetAxis("Horizontal"), 0.0F, Input.GetAxis("Vertical") );
             velDesired = transform.TransformDirection(velDesired);
         speed = walkSpeed * ( bool2Int( Input.GetKey(KeyCode.LeftShift) )+1 );
             velDesired *= speed;
         //Get current velocity
         velCurrent = rigidbody.velocity;
         //Get the difference in between
         velDifference = velDesired - velCurrent;
             velDifference.y = 0.0F;
         //Apply the difference to body 
         rigidbody.AddForce(velDifference, ForceMode.VelocityChange);
         //Get if want to jump
         if ( Input.GetButton("Jump") ) {
             //float velJump = Mathf.Sqrt(2.0F * jumpHeight * gravity);
             velCurrent = rigidbody.velocity;
             rigidbody.velocity = new Vector3(velCurrent.x, velJump, velCurrent.z);
             //onGround = false;
         }
     }
     else {
         rigidbody.AddForce( new Vector3(0.0F, -gravity * rigidbody.mass, 0.0F) );
     }
 }

Everything is fine, I´d like to tune it a bit. I remove "onGround" check in the first if, because I want to move it in the air as well. It works, but if I addForce and a vector which is a direction to my Rigidbody by another collider when I´m in air, I can´t move it anymore. Does anyone know where the problem is, please? :)

Thanks JarPet