it happens.
either, everyone didn't click all the switches at the same time and/or u didn't cleared all the areas properly (kill moar jumpers/seacycles/grunnies).
if you've done all this, and clicked all the switches at the same time then it's just a glitch.
last time i smebbed (yesterday) we had to click 4x's before the glitchy run let us in. again. it happens.
*note:
this can also happen with queen lorelei before entering Underwater Ledge and the grunny sub-commander before entering Sealab Compound. these battles are based on the number of kills you cleared before getting to those mini-bosses. you have to go back and clear what u missed, to reset their HP.