Sounds like the gearbox or the motor itself. On my bedroom slide the motor is/was mounted to the wall under the slide. You can see it if you lay down on the floor and watch it go in.
I have had two slide issues on my trailer. The first was my kitchen slide that was basically popping once or twice every time I slid it in. You can replace the motor for about the same price as the gearbox AND motor so I did both. There are gears in mine that had some teeth broken off. I had a mobile tech replace the parts.
My wardrobe motor slide literally pulled off the wall because it was held on with wood screws. The same tech made a metal bracket that he attached to the trailer frame because he thought new screws in the wood might pull out again.
It could also be out of alignment which requires adjusting the cables. There is literature out there that shows how to do it and I believe one of the members posted here too.