I have been doing this speaker placement method for years, ever since hearing it at RMAF 2007. It is hard to do however, and it took me about 10 years to get it right. And then I sort of wondered why it took me so long.
That locked center image is when you have properly done it, otherwise you are just close but not really there. It is tricky because the locked center image occurs with just a small movement of the second speaker in the procedure.
The Colorado Guy YT video is pretty helpful for certain aspects, and it is recommended. The step guide in AudioAfficianado is very very good.
An alternative way to get the locked center image is to use a mono recording. When you have the locked center image, that image stays in place as you move around the room. It is easier to hear than any stereo recording.