did you use cones or pellets?
whole flower hops will just float around the top mostly. hooking up to a carb stone would work, but that will be in the BBT, which isnt a bad place to dry hop if you can leave it in there for a few days. I think it works best when the beer is still at 20C, perhaps its there but the freezing temps are hidding the effects.
I think 3.5kg will be a good amount in a 15bbl tank.
You could also weigh the bag with something heavy like a few Clamps and tie it to the spray ball or something.