Relationships and forms - not displaying multi selection dropdown - but rather a text box!

I have 4 tables…

Users
Impacts
Stakeholders
Milestones

Relationships are:
Users can have many impacts, stakeholders and milestones (but only 1 user for each type)
An impact can have many stakeholders
An impact can have one milestone.

So in the Impacts table we have

  • Name
  • Description
  • User (relationship: only one user)
  • Stakeholders (relationship: allowing multiple stakeholders)
  • Milestones (relationship: only one milestone)

I have created a form to create a new impact. In the form everything is ok apart from the stakeholders component. It seems to display this as a text box. Milestones is correct and only allows you to select one of the milestones.

This must be something I am doing with the multiple options on the stakeholder but how do I get it to allow me to add any (more than one) stakeholders. Been searching but not found anything obvious! The database looks correct and I can have multiple stakeholders for an Impact.