Not sure if this is in the right place, but could really use some help

My group is working on a10s and we're having problems with the adds

https://www.youtube.com/watch?v=_mXdZvo70KE

skip the around 9:20 when the adds drop

We made it to enrage once, but the last few runs the adds just aren't dying to lb3 anymore and we can't figure out why

from the looks of things they're under the affect of the vuln up when the lb3 hits, which should kill them, but they're still surviving

Ignoring the little issue with aggro at the start of that phase, we could use some advice on how to better position/kill the adds

any advice helps, thanks in advance!