Can't check some of the checkboxes in fillable form once it is emailed as an attachment.

I created a form in Excel, converted it to PDF, and then made a fillable/interactive form.  Once completed, I tested the document to ensure that I was able to insert text and check all of the checkboxes, and it worked perfectly.  I then emailed the form to myself to check that it was going to work when I sent it to clients, however, I was unable to check only some of the checkboxes.  I could insert text in relevant fields.  I have checked all the troubleshooting tips but am at a loss to figure out why this is happening 'only' after sending it as an attachment to an email.  Any thoughts or suggestions on why this might be happening would be greatly appreciated.