I think no one mentioned that we have to tilt up and down, left and right and so on the drone when calibrating the IMU.  I don't see any video explaining this either. On the other hand, I read many users unable to calibrate just like me. I stucked in the step 2 for two days due to this lack of information. Just make sure you follow the pictures shown in the guideline.

P4, on iPad Mini 4 using the latest DJI Go App 4.

I'm one of those goofballs who put their drone in the refrigerator for 15 minutes before doing the IMU calibration.  I DO put the drone inside a plastic garbage bag though in an effort to keep condensation from forming.  I've done this a dozen times with no issues and the warmup period is dramatically reduced when prepping for flight.  

Makes a huge difference if flying in the winter too.  Before I tried the cold calibration, I was trying to fly in bitter cold weather... it seemed like it took 10 minutes for the IMU warmup.  After that I tried a cold calibration... dramatically faster IMU warmup.