take the sling door off and look at the rollers. they can be replaced. I suspect the "wheels" are gone or damaged.

I agree with sdello, it's probably the wheels. they get worn and need replacing after a while, mine have just gone again and need to be replaced as well ;)

I'm having the same problem.With the sliding glass doors opening hard to open. We have taken down the door. (Which was very diffacult.) And replaced the rollers. The store did not carry or make this roller so they sent a replacement. Which still did not work. The door is still hard to open.
I would like to replace the door. But can not find these places that they talk of on TV. "surpluse Or Discount" stores.

Check that the header is not settling on the tops of the doors. My doors were getting pinched.

I agree with the roller replacement. We have a dual-pane door that is 4x8 feet & very heavy. I have to take it down annually to clean & relube or replace the rollers. They get full of dirt from the wind and hosing off the patio. Home Depot & Lowes have exact replacements but they are way too flimsy for the weight of this door. I plan to fabricate my own replacements using industrial grade, sealed roller bearings.

THANKS TO ALL WHO REPLIED. WILL STUDY FURTHER AND SEE IF THIS IS TOO DIFFICULT FOR US OR IF WE WILL HAVE TO CALL IN A "MR fIXIT"

The rollers of many doors are adjustable. Take a closer look at them and see if there are any small screw driver or allen wrench ports.

Believe it or not, you aren't supposed to use products like WD-40. Use spray-on furniture polish. It has wax in it for easy gliding and does not collect dirt like oily lubricants do.
Ed

On many sliders there is a screw that will raise or lower the door. It may be sitting too low and is scraping