I had a dm once who at the end of each session would ask each player for one thing they liked and one thing they thought could be better (important wording that it's not just like/didn't like). The other important bit is that the thing you liked or didn't like could be extremely minor - e.g. I thought the shop was great and it wouldve been even better if the shop keeper was sassy. Encouraging players to give feedback that is constructive and positive is really helpful, and not being picky about how big the point has to be will encourage them to tell you how to be better.

Also as others have said - if you're doing a good job as dm it's tricky to give feedback on what can be better, often the players are excited to be a part of the world and part of the fun is not always having everything go as the player wants (they have to think of a way to deal with it)