Check all fields in the form that are marked as 'required' for validation. If the field has a validation type set, you must make sure that you have an accompanying validation warning for that field.

A common example of this would be for text fields using date format validation. If you require that a field like this have data entered in the format M/D/YYYY, but the site visitor fills in the field with '02/05/95', the form will not submit (because the date is not in the correct format), but if you don't provide a validation warning message, the user will not know why the form did not submit.

You might enter a validation warning message like "Please enter a date in the format M/D/YYYY (ex: 06/15/88)"