Bald-Faced Hornets Still Around After Nest Removal—What Am I Missing?

Hi all, I’m a pest control owner and recently removed a bald-faced hornet nest from a customer’s tree, pretty high up. I bagged the nest to get most of the hornets, then hit the area hard with delta-methrin (Atticus adjourn) using my backpack mister, following the label’s strongest recommended dose. I sprayed all around the nest site and nearby branches to catch any foragers. Came back a day later, and about 20 hornets were clustered on a different branch—not where the nest was but in the treated area. Why didn’t the delta-methrin take out all the foragers? I thought it’d kill them on contact or with residual effect, but these guys are still hanging around. Is the insecticide not strong enough, or does it take days to work? Also, what’s the best way to treat the area post-removal to prevent this regrouping? Should I switch to a different insecticide, like lambda-cyhalothrin, or stick with delta-methrin and add traps? Any tips on setup or follow-up to make sure these hornets are gone for good? Thanks!