Hi,
You might have a set of "Hi Performance Racing" shoes, ie; a set that has a high temp working range, great when you are road racing, but no good when the shoes and drums dont get hot enough, when you ride VMX.
If you are getting new shoes, or getting brakes shoes "rebonded" get the softest linings you can, yes they wear out quicker but they work better when cold and when they are wet,
cheers