I had the same situation with my Powergear operated bedroom slide on my old coach. Would go out a few inches and then not go further. It would come back in OK. I raised the bed and activated the slide. When it stopped, I could still hear the motor running. reversing direction, it came back in. I pulled the motor/gearbox assembly and took apart the gearbox. One of the gears in the gearbox had 2 broken teeth. When it got to that point going out, it would "skip"... when reversing, it would just catch the remaining teeth and come in ok. I contacted Powergear, they said I needed to buy a whole motor/gearcase. I searched around and found someone that had a broken motor unit and he gave me the gear I needed. Worked for years afterwards. As a temporary work around, when you are extending the slide and it stops, give it a push and if it's a broken gear, it may catch and extend the rest of the way.
Here's a pic showing the missing 2 teeth on the larger gear.
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