If it isn't a problem, could you upload the brush? My photoshop express will not let you edit brushes to get it to react in size to pen pressure (how its small to big to small again)
I think it does, since it works for ocapacity on the 19px brush I use for sketching but it just works for siza on all the other brushes or something D:
I just use a standard round PS brush - with the shown settings