I know how you feel. When I first moved out, I snagged an apartment. It was great for the first 2 months. Then a bunch of douche-bag teens / early-twenty-somethings moved in upstairs. My apartment would constantly smell like weed and worst of it all was the most concentrated part was in my closet with my clothes.
Luckily they got evicted 3 months later for destroying a closet door. However, 8 months later a really shitty family moved in and it smelled like weed again.
You are in a sticky situation because you don't know who exactly it is. My guess though -- it is the person right above you. In my old building there were pot smokers to my left, top-left, and right. It never smelled until the people directly above me smoked. Maybe that will help narrow it down for you? How long have you been there? Have you watched people move-in/out from your building? You can try to eliminate people by thinking that the new tenants aren't the cause or think if it started after someone moved in.