after many weeks of being frustrated by this bug i did a little digging in the forums & some analysis of our platform phase, I've found out what causes the shield stacks to not fall off
It appears that the devices(the things you destroy to drop his stacks) are set as a chain, from the 1st platform down the corresponding side, so if you drop to the 1st platform & kill that one(activating say the left chain) then some of your ranged shoot the opposite one(the first platform down across the gap) it triggers the other (right)chain, meaning any devices you kill on your way down(the left chain) no longer function. that's why most see it stop @ 3, it drops off from the top 2 devices & then is "locked out".
Easiest way to fix it is to not shoot any devices on platforms you're not jumping to. solved the issue for us for a whole night of attempts.
Hope this helps