The slide with the least resistance will move first. I have 3 slides, sometimes one will open part of the way, stop, another will open, then the first one will finish. It could be that the hydraulic cylinder for the large slide has failed internally or the pump is not producing enough pressure to open the larger slide. Also if the battery is weak. the pump will not pump enough to move the large slide. With shore power plugged in, it still needs a strong battery. If you can get close enough you can use jumper cables from your vehicle to see if a better battery will do the job.
