“Add to Cart” field shows product variations instead of individual options

This is probably a very novice question, but I'm working on a website that uses Drupal Commerce. There is a field defined for the product display called: Product variations which is of the format Add to Cart form.

On most products, this field correctly shows a combination of size and color dropdowns, whereas for some products, it just puts all products variations in a drop down and shows that instead of the color/size combination.

Any idea what might be going wrong or how to debug this issue?

Wrong:




Correct: