Sliding closet doors are also called bypass doors, whereas bifold doors consist of hinged panels hanging from an overhead track. Bypass doors can also hang from an overhead track, but heavier ones -- which often support mirrors -- usually slide along a floor track. When either type sticks or doesn't close all the way, the remedy is a simple one: Adjust the rollers.
Adjusting Hanging Doors
If your bypass doors hang from an overhead track, each door is supported by a pair of rollers that glide along the track. The distance between the roller and the top of the door is adjustable. The adjustment screws often loosen, and sometimes it's because the door material is too thin to hold them. If it's just a matter of readjusting the rollers, you can do that while the door is hanging, but you'll have to take the door down to reposition rollers that are coming off.
Check the floor guide and tighten it if necessary. If it is loose or missing, the doors can swing out, and that puts pressure on the screws holding the overhead rollers.
Adjust the rollers on the back door of a bypass pair first. Close it all the way and examine the gap between the edge of he door and the jamb. Put a shim under the front or back of the door as needed to make the edge of the door and the jamb parallel.
Open the front door and go in the closet, wearing a headlamp to help you see. Locate the adjustment screw on the roller on the rear door that is not in contact with the track. Loosen the screw with a Phillips screwdriver and move the hanger until it is in contact with the track, then tighten the screw.
Remove the shim and, while remaining inside the closet, close the front door. Place the shim underneath it to make the edge of the door parallel to the jamb, then adjust the overhead roller.
Take the doors down to a reposition roller, if necessary. Unscrew and remove the floor guide, then go outside of the closet and swing the bottom of the front door toward you while you lift the door to unhook the rollers from the track. Remove the rear door -- if necessary -- in the same way.
Unscrew and remove the loose roller, move it about an inch toward either side of the door and screw it back in. Replace the door and adjust the rollers using shims.
Adjusting Rolling Doors
If your doors slide on floor tracks, each one has a pair of wheels that you should be able to adjust by means of screws in each edge of the door. Look for the adjustment screws -- which may be recessed into the door frame; turn each one clockwise to raise the roller on that side of the door and counterclockwise to lower the roller. Adjust both screws as needed to make the edge of the door parallel to the jamb.