Submit not working (sometimes!)

Hi! 

I seem to have this strange problem. I've created a quiz where the user clicks an in-slide submit button to submit their answer. When I test this, every so often the submit button doesn't work at first. It eventually does submit every time, but sometimes this is after I have clicked the button two or three times.  It's not every slide, and the affected questions seem to change, but I can't seem to figure out why it happens? Is it just an unavoidable bug in the system? Or is there something I could do to fix them all and get them all submitting on the first click every time? I know this sounds minor, but thought I'd ask in the off chance someone knew a solution. 

I've attached the Storyline file. 

Thanks! 

 

Fraser