Firstly, make sure that the modal fields are wrapped inside the <form> element. I could be that your modal HTML is not inside the form tags which would explain why it isn't being submitted.

However if this is not the case and the fields are within the <form> element then chances are when the modal is set to display:none; the form ignores those fields and treats them as disabled. If this is the case you would need to pass the values of the modal fields into hidden inputs inside your form, and use the $.modal.close callback to pass the data that way.

I'm not sure if the second one applies but something in the back of my mind tells me this could be the case.