Most likely it is because you are failing to really squeeze at the top of the motion. If you just go through the motions you will not obtain the goals you are looking for. Try doing some high rows and underhand barbell rows and really focus on squeezing your shoulder blades together and maybe throw in a rest pause or drop set to really get the blood in there.
snatches are great for that. even snatch grip deads and even another i totally forgot about, farmers!Snatches gave me the trap growth I ever experienced.
make sure your squeezing them together behind your neck if that makes sense
Can you post a photo? It's just that while I have good traps, I'd feel like an ass giving advice to someone with even larger traps than mine who wants them to be bigger, you know?