Can't submit the form?
Do you have an Opt-in or Terms and Conditions field that is mandatory? Check to make sure the colours of the form aren't obscuring when the checkbox is ticked / unticked, and that you have set some appropriate text copy for the "What message should we show if someone doesn't tick to opt in?" field. This can make it appear that a form can't be submitted, but in reality the mandatory checkbox just hasn't been selected.