If you are sure that nothing is underneath it blocking the retraction , see what brand it is and do a search of possible causes.
If its grinding, then it's a metal to metal issue i would guess some kind of binding/alignment issue of the track , rollers or the bolts came loose on the motor (you'll see the motor move). One person to operate it and the other checks the mechanics of it using a good light.
Less likely: Sometimes it's the factory install that's the problem and being you have not used your slide for years, that might be the case.
Could be an alignment issue with the track itself or not being square from the factory.
The following link has multi-pages and you might find the answer that pertains to your slide condition.
http://www.crossroadsowners.com/forums/f25/slide-wont-go-back-in-9125.html