It sounds like the slide is out of alignment. The gap should be equal.
We had a similar situation with our largest slide, except for us the slide would not come fully in at one corner. Like yours, there was a large gap on one side, but almost no gap on the other side. As the slide came it, it would bind on the side with little gap. Turned out the tongue was jacked up too high. When I lowered the jack, the slide came into alignment better and it closed properly.
Some slides can be manually adjusted, check the manual.