First of all, welcome to the Keystone Forum.
With the little bit of information you've given, it could be a number of things, but I'd start with cleaning and lubricating the seal gaskets around the slide and making sure it is sitting square in the frame of the trailer.
I'm not sure what system was used in your RV to actuate the slide, so here is a link to the LCI page with all the Lippert slide mechanisms listed:
http://www.lci1.com/index.php?option...174&Itemid=103
I'd start with the troubleshooting guide in this link:
http://www.lci1.com/images/Flyers/Ow...ideout-web.pdf
In addition, Lippert has a long list of solutions for different issues on their slides. It can be accessed here:
http://www.lci1.com/index.php?option...191&Itemid=121
There are also specific cleaning and grease application instructions on the LCI solutions page.
It should at least get you started.
My thoughts would be that if it only is "sticking" going out, and it works OK going in, there's something with the way it's "parking" in the frame of the trailer. If it's "sticking" both directions, then there's a host of other things. That's why more information is needed.