For those of you who don't remember Hunter Fest「狩りの祭典ハンターフェス Kari no Saiten (Hantā Fesu)」is a technique that allows Ban to steal the physical strength of every living creature within a hundred foot radius of himself. However, as noted by Merascylla, there is a limit to how much strength Ban can rob a person from, as seen in his encounter with Galan. Once the limit is reached, Ban is incapable of robbing the person of more power as it puts his own body in danger.
So my question is why didn't Ban use this ability when the sins where fighting Gray Demon Hendrickson? I don't think it was a matter of he couldn't because there was no training session for Ban between then and when he fought Galan, he was beating Galan senseless so couldn't he have done something similar to Hendrickson?