are you tightening down the torca clamp and front strap first, then the rear? if so, you may be putting extra stress on the front strap by doing this.
because its tough to balance the pipe underneath, hold it, and tighten, some will secure the front torca and strap first, then the rear. this ends up stressing the front.
i don't have a manual handy but the best way i found (along with keep spare straps around, i've busted all three types) is to use a floor jack to hold the pipe tight up against the mount points under engine, THEN tighten down the straps a little each at a time, torca clamp last. some RTV on the header -> pipe connection is a smart idea too.
over torquing the straps is common as well, they don't need much to hold things secure. you can actually bend your pipe by tightening to much as well.
hope this helps.
EDIT: oh, and dont reuse the straps (yeah, yeah, i know lots reuse them, i used to too) they're one-time use only. yes, this sucks and its expensive if you swap pipes a lot but they will fail.
