There are three possibilities that may prevent forms from being used as subforms :
- The form already contains another subform.
- The forms may have data in them. Only forms without data can be attached as subforms.
- 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.