Servos or Steppers?
If steppers, you are losing steps.
If servos with encoders on the motor, something mechanical is slipping. For servos, the Galil gets it's feedback from the motor encoders. If the numbers are right, then the motor shaft went through all of it's paces. But the screw may not have! Check your couplings and/or pulleys for slippage on the shafts.
Steve