Respondents can't submit form

Hi,

I have a form which customers submit to request lessons. Recently customers have reported they cannot submit, we've had over 100 responses over the last month or so but I've tested it myself and at the final stage when clicking submit it responds like a compulsory question isn't answered but doesn't come up with any error.

I've removed the "required" from all questions and it seems to work. There are multiple sections which are not all completed due to branching so assuming it's related to this