commerce product display showing only one product when multiples selected
Check the Display settings for the Product Display type in Structure -> Content -> Product Types, and make sure that the Product field is set to be visible on all view modes.

Set it to "Add to Cart form" and it will display a choice of all applicable products.

Note - I had a duplicate "product" reference field on products which confused the issue, and this was what was displaying the single "add to cart" button.