How to ask for useful critiques?

As a GM, I strive to make the best experience for my group and adapt and improve my style and skill. I usually do this by live observation and asking for inputs after the session.

My problem lies in after session Q&A. Usually I ask them “Do you have fun? Which part is fun? Which part is not fun? How can it be better?” and they answer generally “Yep, it was fun. I like every part of it. I think it’s fine.”

I don’t want to look for problems that aren’t there, but I feel like I’m not improving the group’s experience this way. There are some parts where they are distracted enough to play with their phone – they are not engaged enough, so it’s obvious I can change some things, like pacing, so they can enjoy the session more.

I want my players to give feedback on every session. How to make them do so?

I’ve tried to make the questions as guiding as possible and as brief as possible, but they stick with the general “it’s fine”.