Are certain bees more resistant to SHB? The reason I ask is that we were in the hives doing our last check for the season and found a lot of beetles in three hives and none in a fourth. I will say the last hive had the most bees, so we thought it might be a factor of the added bees keeping the beetles at bay. I think we are going to put some slide in oil tray traps before winter and try to knock down that population a bit.
I think that some colonies are more aggressive towards the shb than others. However your observation may be related to other factors such as the one you mention ie the number of bees in the hive. Other factors could be the amount of free space in the hive allowing more room for the shb to hide, the sun/shade factor etc.
In any case I would treat as urgent getting those oil traps into the bottom board
Mick