Where are you feeling the "locking" happen, what else can describe. Does the ankle want to turn out? Does it not want to go out over the toes properly? If you can manage a few dollars I would suggest getting Becoming a Supple Leopard - Kelly Starett. It is an AMAZING book that is easy to read and comes with many pictures on how-to's for mobility.
I have a similar issue. I have sprained my left ankle and twisted it MULTIPLE times while playing soccer. It now has mobility issues and does not have the dorsiflexion it SHOULD have.
On a side not your position looks pretty good. It looks like you are arching your back a little too much.
Look at this photo and notice the differences. Try and emulate the vertical back.