More than likely you couldn't move because you had a jump available (or a chain jump) that you weren't noticing. As per what is supposedly checkers tournament rules, you must take an available jump. If I had more time I'd've added indicators for which checkers can move and where but I ran out just after finishing the core game. Probably not a bug, I've been able to beat it a dozen times without anything breaking, but definitely something that could be improved.
Thank you for the comment!