The best scenario is when the soundproofing treatment is on the sound-producing side if the partition. This reduces sound flanking. This isn't possible in your case, I understand. Also there is sound coming through the walls to some degree. That's the nature of sound transmission. Sometimes its a lot sometimes a little.
In either case about all you can do is look to add mass to your floor and possibly damp that mass. More effective solutions take up more floor height which causes issues with doors opening, etc.