You can configure which question the bot asks if the entity is missing. Please make sure you've trained entities, and your entities are added to your intent (else it won't show in the specific intent flow).
Example, the bot wants to know:
Departure time
Departure location
Arrival location.
You set these (3) entities in the block (with a max of 10)
This example, the block detects all entities and continues.
In this example, the block replies "please tell me the arrival location"
and continues to ask "please tell me the departure time"
if the block detects all entities, it continues
in this example the block replies with "please tell me the arrival location"
if the block detects all entities, it continues
*this block can't be used in the welcome intent