There are three possibilities that may prevent forms from being used as subforms :

  1. The form already contains another subform.
  2. The forms may have data in them. Only forms without data can be attached as subforms.
  3. The forms have already been used as subforms in other forms.

If in your case the issue is #2, you must either:

  • use a copy of the form or 
  • delete the forms data.

We recommend making a copy of the form and using this copy to avoid losing valuable data.