Wooden sliding patio doors that sit on tracks with sliding rollers tend to be vulnerable to grease and dirt build-up and wear to bearings and wheels. If the door is starting to jam when you try and open it, it's time to repair or replace the rollers. Some doors have rollers on the top or bottom and some only on the bottom. Heavier outside doors are seldom hung only from the top, so start your repair work from the bottom and work your way up.
Inspect the edges of the door frame, especially the roller track at the bottom. Often a bent frame causes the door to drag. If the roller track frame is jamming the door, it will need to be bent out of the way. The track is usually made of aluminium. Insert a screwdriver and pry it away from the door. Be careful not to scratch the visible part of the door. Use the heavy lineman's pliers to straighten any bent part of the roller track at the top or bottom of the door frame.
Remove the door from the frame. There is a Phillips head screw at the base of the door just above the roller track in the side or edge of the door near the rollers. Turning this screw to clockwise will lower the rollers, turning it counterclockwise will elevate the roller. Lower the rollers. As you do so, this will lower the door at the top. Have someone hold the top of the door or make sure you don't lower the door so it's top slips out of the upper frame and falls out.
Lower the upper rollers by twisting the adjustment screws clockwise. When the lower rollers are all the way down, grasp the edges of the door and lift it up into the upper frame. As the lower frame clears the roller track, swing the bottom edge out of the track and lower the door out of the frame. Set the door on the sawhorses.
Check the lower wheels to see if they are damaged or flattened. If the rollers are jammed or the brackets are bent, you'll have to replace them. If they are just worn, you may be able to get away with swapping them for the upper rollers -- especially if you can't find replacement wheels for them because the replacement parts aren't available.
Unscrew the wheel brackets and remove the wheels and brackets. If you plan to swap them with the upper wheels, remove the upper wheel brackets as well. Clean the tracks, wheels, brackets and the frame with steel wool, soap and water. Lubricate the wheels with silicone lubricant. Silicone lubricants don't attract dirt as much as petroleum-based lubricants. Screw the replacement brackets into place on the underside of the door. If you are swapping the upper wheels, they'll usually be the same bracket. If not, you won't be able to swap them and you'll have to live with the old ones or find new ones. Check shops that supply doors and windows.
Retract both top and bottom rollers. Turn the height adjustment screw counterclockwise with a Phillips screwdriver. When both wheels are fully retracted, slip the top of the door in the upper track, lift and set the lower track in place. Get a helper to hold the door upright while you extend the upper wheels above the edges of the roller track by turning the adjustment screw clockwise.
Raise lower wheels by turning the adjustment screw clockwise. Alternate extending the top and bottom wheels till the door is secure between the rails and rolls easily without binding. If the other half of the sliding door slides too, repeat this whole process with that door, even if it doesn't have a problem. Preventive cleaning and lubrication can help prevent you from having to replace the other set of roller wheels.
Try to find new replacement wheels. They may be available online or at a local window and door supplier. Only swap the upper and lower rollers if you absolutely have to.
If the replacement part doesn't fit, don't force it. You may not be able to get the door to work properly if you do. Get the right part. Have a helper hold the door while you are adjusting the roller height to prevent the door from dropping out of the upper rail and falling which could cause injury.