Some tote bags have a waterproof coating that is sprayed onto the bag after it is assembled. This could be preventing the discharge ink from penetrating into the fabric of the tote. Also, certain thread manufacturing processes are not discharge friendly. So you could've gotten bags that, although they are cotton, the discharge ink is unable to discharge the thread used to make the bag.
I've also found that regardless of the fabric, certain colors are not friendly to discharge inks. Mainly green colored fabrics. I've always had bad luck discharging green.
If you are seeing a small amount of discharging on the bags, you could try increasing the concentration of ZFS in your mix. That might increase your likely hood of the bag actually discharging. I've had certain cotton products that needed 6% ZFS before seeing results in discharging.