Mark questions in a question pool as "mandatory" to be pulled every time.

The "Include In Shuffle" column has options for how to handle each question. 

Here's what the User Guide says: 

• Use the Include in Shuffle column in the question grid to decide whether each question will be Randomly displayed (which is the default option), Never displayed, or Always displayed in this particular question draw.

Here's more about drawing from a Question Bank: Storyline 360: Drawing Slides from a Question Bank