Hi BarbandFrank,
Since the other slides are working, your hydraulic pump and electrical system are working.
On the failing slide, if the pump makes noise when you press the switch to operate the slide, then the problem is likely with the valve that opens that slide, or perhaps the wiring to the valve. If there's no pump noise when operating the switch, you probably have a switch problem. Sometimes rocking the switch back and forth will get things going - that may be a sign the switch is starting to fail.
If you think it's a switch problem, you could try swapping 2 of the switches. If a bad switch, the problem will move to the other slide. If a wiring or valve problem, swapping switches won't make any difference.
BUT, before experimenting with the switches/wiring, use your phone to take a good, clear picture of the backside of each switch so you can get wires back in the right place.