“Of whom we have many things to say,  and hard to be uttered,  seeing ye are dull of hearing” . (Hebrews 5:11)
In a communication with a child,  it becomes very difficult to talk about certain things that you’d talk about with an adult.  Not because you don’t know what to say,  but the ability of the child to understand the complexity of matters.
This happened to the author of Hebrews,  when it became so tough for him to say many things to the church. They were simply “dull to hear” the revealing things to be uttered.

Just as it was,  so it is today in this present age.
To be dull to hear is to be slow to perceive through the spiritual senses (sight, hearing,  touch , smell and taste). It is a mother problem that breeds several problems in a believer’s life.

 ” O fools, and slow of heart to believe all that the prophets have spoken” (Luke 24:25)
Some of the believers at that age were slow of heart to believe. In other words,  they were dull to perceive with their hearts the fulfillment of the scriptures right before their eyes. And the Lord referred to them as fools.
“and beginning at Moses and all the prophets, he expounded unto them in all the scriptures…” verse 27
Our Lord applied a solution to their very problem.  He expounded the scriptures to them to take away their dullness of hearing.

Teaching or expounding God’s word eliminates dullness of hearing.  It sharpens the spiritual senses to make you apt to perceive spiritual things.
“and their eyes were opened,  and they knew him… ” verse 31

The principle applied in verse 27 produced an expected results. Their spiritual senses were now alive and active.  Glory to God!