Stan......I can't tell you how to adjust your particular slide, if you want to do it yourself, you sometimes have to study it a little and see where it adjusts. You don't say if the gap is at the top where the slide room is sitting low, or it doesn't go in all the way.
If it's low, there is usually an adjuster under the slide (looking from outside) that lifts the room up at the outer edge. If it's not going all the way in, check for obstructions as mentioned. If no obstructions, make sure that that you haven't broken a tooth off at one end of the slide rail that is not going in. They often have a shaft that drives the side on rails at each end. Make sure the shear pins that connect the shaft are not bent or broken. Lastly, there should be an adjustable stop under the slide that stops the slide when it gets to a certain.
Your coach make not work exactly as I described, but may give you some ideas on what to look for if you want to fix it yourself. Recently, mine was binding and I noticed that one end had dropped about a 1/2". Once I adjusted it, it started working fine.
Don & Mary
2019 Newmar Dutch Star 4018
2019 Ford Raptor