My 2004 Flagstaff 5th wheel had the same problem. We had to gently push to get the slide to go out or in. I purchased a can of slide out lubricant at Wal-Mart and sprayed the teeth and gears under the slide. We worked the slide a few times and it seems to be OK.
Are the seals sticking? If so they need lubricatede as well. I use a silicone lubricant on these to keep them from sticking.
2006 2500HD Chevy 6.0 Gas Ext Cab Short Bed
2004 Flagstaff 8528BHSS 5th Wheel
Dear Wife, two daughters and four dogs