I am also experiencing this on a '17 Sierra Denali with ~33k miles. I took it to a dealership last week to get them to take a look at it. They didn't seem to have much interest in dealing with it, but also advised me that these running boards and their components are on back order due to COVID-19.
In the meantime, I have started to try to locate some of the individual parts to try and resolve this myself. I haven't made much progress on that front. At least on mine, it seems like the motor is weak for the Drivers side board. It extends slowly, makes a 'rubber on rubber' squeaking sound while it is in motion. Passenger side extends quickly. This doesn't help with getting out of the truck, but getting into the truck, I have noticed that if I help the board extend by putting my foot under it and pull when I open the door, it extends just fine and remains extended as it should. Similarly when I extend to the bed.
I took a photo of the motor for the part numbers, but my searches haven't come up with anything. Here's an image of it: https://photos.app.goo.gl/E13uPgdav4bwsJRE8
Here is also a video showing the extend-retract issue: https://photos.app.goo.gl/BgZFMmbcJvuuP5Hr6
Edit: Ah, I see this was a necro thread. Sorry for piling on. Still relevant information, since I have not been able to find much at all on this topic.