hey guys iam trying to build a quadcopter,and i have four dt750 motors and four 20A escs to begin with.And before connecting the escs to flight controller,i wanted to ensure if all of the four motors are spinning or not.So i connected four escs to a 4pin servo tester,and guess what?,one of them is not working and instead of spinning,it is vibrating.There is nothing wrong with escs.I need your help and please give me an explanation.

Post a video.

Check for loose connection,  Also are the motor creating any sound, swap esc and check

Sorry for the late reply,i even tried using a different esc and still there is no difference.how can i make it spin again?

I will upload a video soon