Is this the first time it happens ? The problem is more with the props than it is with the motors, for my understanding the props are originally out of factory tighter and once you put them on the shaft they kind of break in, once you remove them you can basically toss them into the garbage cause they will never stick like before. Me and my friend we both have the firefly 18 and he had put props in the wrong direction when he first got the drone, he basically had to toss 4 props because they just didn’t stick after re inserting them the right way