There are two types of slide outs. One is hydraulic and the other is mechanical. If you have a hydraulic system, you may need to add in more hydraulic fluid. A lack of fluid would cause the slide out to only be held open/out as much as the amount that the fluid can fill the fluid chamber. If there’s not enough fluid in the system, then the slide out will not be forced or held out as far as it could be. So check the fluid level if you have a hydraulic system.
The mechanical kind has little pinions that click into tracks that roll outward as it pushes the slide out. If you have a mechanical one, you may be pushing the slide out to a small distance between the pinion’s nubs and it rolls back to click into a closer notch on the track. If this is the case, perhaps just push a little farther to get the next notch.