I built a racer drone. The problem I have is that 3/4 times when I arm it will flip over. I took off the propellers and synced the esc but I tried testing all the motors on betaflight and motor 1 (right back) will not spin. I don' t think it is the esc or my Emax red bottom motor because the few times it does arm correctly it flies perfectly. Any ideas what is wrong?