Its likely that the new shoes created less clearance to the drum ( ie, your old shoes were worn down) This would cause it to engage quicker than the old shoes. Also, the springs may be too soft for the newer, thicker shoes. Its also possible that the idle needs to be adjusted down so that the new shoes dont engage as quickly. I know it sounds crazy, but new shoes that are not worn out will be "thicker", therefore heavier. Heavier shoes get "thrown out" quicker than wornout lighter shoes. The new heavier shoes also need a stiffer spring than worn shoes, or else they will engage much quicker. Of course, this is all relative to the degree of wear on the old shoes, but it is still a factor to consider. Just my .02
Mike