I have the same problem once in a while with my superslide.
In my case, the slide has to climb a ramp for the first 6 inches or so, which makes it much harder to move for those first 6 inches. The slide rides up a piece of low-friction plastic that is across the whole width of the slide. This is the feature that is advertised as the "flush floor" of the slide.
When fully open, my slide has dropped down a little lower than it was when new, indicating that something has moved or worn somewhere. This is what is causing mine to need a little push to get it going. I will need to check the slide alignment and wear.
Steve