"I have a 20 year old sliding glass door"

A window or door company is not normally going to replace rollers and or service a 20 year old door. You would need a small contract or handyman. Dependsing on the brand parts may or may not even be available. Roller may not be able to be changed as we do not know the condition of the attachment points after 20 years. BUT me personally I would say you have gotten good use out of it and it may not be worth spending the dollars to attempt a repair. Do you know the brand? Pictures? Vinyl, Aluminum, Wood?

How handy are you or is anyone in the home? They can be serviced, but that is not a task that most professional companies will want to take on. Its tough to charge enough for a service like that to make it financially viable (in the case of a company vs. a handyman) as well as honor any service on the unit after the fact.

Its a bit of a liability exposure trap for most companies. Company A services the door and something else on a 20 year old builder's door fails down the road. Many instances like that will net the customer looking at the company as if they are on the hook for continuing to the fix the door because they now "own" it.

If you are handy, there are a couple of places that you can likely source the parts from. Just depends what you want to take on.

https://blainewindow.com/

I had a 20-year-old sliding glass door that also wasn't sliding. I thought it was the rollers. I did several things that were recommended on this website and on the internet to help the doors slide. That would work for a month maybe. Turns out it was actually the frame that was warped and rotted so that the track on the frame couldn't stay straight. I couldn't even see how bad it was until I had it replaced and when it was removed I was shocked at the amount of rot throughout. I'd had no water infiltration, so had no idea. But the fact that the door couldn't slide even after bandaid repairs was the clue.

Mine was a Pella (ugh). What a piece o' crap.