First, check the oil level in the pump reservoir, if it is at the full mark run the slide in/out and recheck oil level. If oil level remains the same you may need to repeat that several times to work any air bubbles out of the system, re-checking reservoir each time the slide is retracted.
Another possibility is lag-time from when the slide is retracted until you release the switch. Hold the switch on retract approx. 30 seconds after you notice a change in pump sound.
I hope one of those work.