We had a similar issue - our "front slide" was a main living room slide where the slides raises up then drops. When retracting it would nearly come to a stop until it raised all the way, then it would fully come in.
Later we'd have to have someone outside push while pressing the retract button.
At first I thought it was because we weren't level enough. For awhile we were OK as long as the unit was level after extending (if the slide side was slightly lower it wouldn't retract).
Then the problem got worse and I realized I hadn't maintained my house batteries. I filled them up, charged them, and the problem went away.
After a few years the problem resumed and I took it to a tech. The old slide motor had gotten tired so I had a new one installed.
Now we are back to being all good.
I hope this helps.