Using custom fields in other custom forms

This seems too simple to ask but I just cannot find the solution!

I have inherited the SA role from a previous employee who created 99% of the custom fields and now I would like to create a new hybrid custom form from two existing custom forms, using the same custom fields.

When I try to duplicate a custom field from form 1 into the hybrid form, the 'apply' box is ghosted. The field is shared sitewide and I am named as Admin.

What am I missing? I feel it is something really obvious.