Observation -

You seem to be following 'guidelines' in the DMG pretty specifically. But then you don't give XP, you use milestones for leveling up.

Is your progression slower than if you were giving out XP as per book? If it is slower then perhaps players the slower progress is an issue.
Also, their is a psychological aspect to getting XP. It is a measurable progression towards going up a level.

You may want to consider switching to using XP. If you want slower progression don't give as much XP out, I have calculating XP as if the monster had one less CR, matches my preferred progression rate.