First, check your slide mechenism. If you have cables, make sure they are relatively tight and not sloppy. This is a very easy fix, lots of YouTube videos showing how to adjust and tighten the cables.
If you have the Schwinteck system, check the motor, and gears for broken teeth. Same with the Rack and Pinion system... check the gears under the slide to make sure no teeth are broken.
Also, dried out slide seals will cause excessive resistance on any system, and will cause slides to operate like you are describing also. Get slide seal conditioner and get them softened up. Putting wax on the slides also is a big help. It makes them slicker so the friction resistance between their sides and the seals is not a great.
2019 Montana High Country 375FL Fifth Wheel
2014 Chevy Duramax HD 6.6 - 3500 Diesel Dully Long bed Crew Cab