Sorry I am late on this post but I have been away from the computer. As a locksmith I have fixed lots of these over the years.
Several things might be the problem.
First it could be an adjustment. At the bottom of the door about an inch and a quarter from the bottom there will be a 1/4 inch hole on each edge of the frame. #2 Phillips clockwise raises the door. The gap should be even top to bottom when you slide the door open a bit.
Second, the rollers could be not rolling due to carpet fibers cat hair and general dirt. To clean and inspect them, slide the door open a few inches and lift. This will allow you to tip the bottom of the door clear and remove. Upon inspection, if the rollers have run flat you can replace them for a few bucks at th4e local glass shop or a good hardware store. Bring the old ones to match them up.
The third thing that could be wrong is that the extrusion that the rollers ride on has worn beyond the point that you can adjust the rollers. If this is the case, you can get a U shaped strip at the glass shop for rollers to ride on.
If this is a double glazed door and it is wide, the door is going to be heavy. It is not advisable to repair these on a windy day and a helper can prevent an expensive oops.