Aleš Lombergar

Double pane sliders are very heavy. First, you make sure the track is clean, then you either lubricate the rollers or replace them. CorrosionX is a wonderful lubricant and corrosion inhibitor… much better than WD40.

At the bottom of slider there are rubber caps or possibly a hole or just a slot, Insert a Phillips scewdriver into slot or hole and feel for a screw head, This is an adjuster screw that will lower or in your case raise door. remember to do both ends evenly and use wax or some form of dry lubricant, oils tend to hold the dirt. Good Luck!

All sliding glass doors have casters onthe bottom side of the door, and these casters are adjustable. (This lets you keep the door square against the frame, when you close it.) Over time, the weight of the door, can cause the adjustment on these doors to change.

How to adjust the door:
Look at the bottom of the door (on either side of the sliding panel) and you should find one, or possibly two phillips screws, on either side of the door. (Sometimes the adjustmen screw is hidden under a plastic cap/plug) If you have two screws, one may look like it is on the outside surface, and the other (usually directly above or below the outer screw), looks like it is setting in a hole. The screw in the hole is the one you want to turn, it is the one used to adjust the caster under the door. The other one, helps hold the caster assembly in the door frame. (On a rare occasion, Ive found the adjustment screw to be an allen head)
Usually, turning the screw clockwise will cause the door to raise, and counter clockwise will lower it. It sounds like over time, yours has lowered.
By the way, you will need a fairly large (probably phillips) screw head… number 2 at least, and more likely a number 3. If you don’t know what that means, its the larger size phillips screwdrivers (I suggest doing this with a hand screwdriver….it won’t take too many turns)
Give those screws on both sides of the door a few turns, and see how it slides… if need be, give them a few more…then… you need to check for alignment.
The easiest way to see if the door is aligned with the frame properly, is to slide it to where it is almost closed….. leaving a very small gap, between the door and the frame…. this gap should be even all the way down the door, between the door, and the edge of the frame.
If the top has a wider gap than the bottom, then the door needs to be adjusted a bit. You can do this by either bringing the backside of the door up, or lowering the latch side… since your door has been dragging, I would suggest bringing the backside up.
If the gap is wider at the bottom, than at the top, then you need to bring the latch side up. Again, this is done by simply turning that phillips head screw in the pocket, clockwise. You can also even it out by lowering the other side.

This is an easy to do adjustment. You can use a screwgun, if you like, just make sure you use the correct size bit, or it will strip the screw head. I find it is quite often, easier to simply use a hand screwdriver, as you can make finer adjustments with it.

Also — to help the door slide, you can lubricate the track (top and bottom). If you have plastic casters under the door, you do NOT want to use any kind of silicon, or oil based lubricant. (This applies for homeowners with vinyl doors and windows, too) The oil base products ‘eat’ at the plastic…. you’re better off using a wax based furniture polish. In your case…. with an aluminum frame, I would suggest that you try a product called Topcote, made for tool surfaces actually, but works well on other metal surfaces too. You can get this at Rockler.com, and Ive provided the link to it, below. Note – Topcote is the table lubricant, that is the one you want…. Dricote is a saw blade lube. For an immediate help tho, a simple shot of Johnson’s furniture wax will help. Furniture wax can also be safely used on vinyl doors and windows.

This really is an easy fix, and should only take you a few minutes.

Have Fun!

Edit: After seeing a number of people here have suggested using WD-40… let me re-emphasize, DO NOT USE AN OIL BASE LUBRICANT, if you have plastic casters… (or on vinyl windows and doors)… it WILL eat at the plastic, causing it to become brittle, and crack over time.