Can you see the contact patch on the shoe - btw, you didn't mix up the front shoes and swap the sizes did you? - you might be getting contact at the leading and lagging edges of the shoe. Find somebody that can do a radius cut on them and have that done. What happens is the leading and lagging edges become like a spring and when more pressure it applied they don't dig in deeper as much as flex a little without letting more of the lining touch the drum.
