Next time, you may wish to put an inert weight (e.g., stainless steel nut or two) in the hop bag and tie a line (e.g., monofilament fish line) to the bag and secure one end outside the bucket or keg (e.g., by looping it around the out post on the corny keg). You'll want to sanitize the weight and line, of course. That should help submerge the bag and get more extraction of hop oils from your dry hops.