I had a similar problem.
The slide wouldn't come in, or came in very slowly, especially getting over the hump at the beginning of movement (the slide must raise up about 2 inches).
I had the motor replaced under the extended warranty, and that helped, but it still had difficulty getting over the hump.
I discovered I had too much "stuff" in the basement of the slide; it was too heavy! If DW gave it just a little push to help it get started, it was fine.
I removed all the Amateur Radio antenna gear I had in the basement (I only need it once a year, anyway), and now the slide works much better.