Please Help!!

My wife and I recently purchase our firt home (a condo actually). We are on the top floor (4 floors). Since moving in our tub drain has gurgled when we flush our toilet. We have notified our strata management compnay who have tried to both snake the stack vent on the roof as well as have it blown out with a high pressure hot water hose of some type. Well the gurgling has only gotten louder. The apartment below us has no such problem with her plumbing. I have no idea what is going on. Everything I read tells me that it's a stack vent but I am wondering if there could be another cause. Any suggestions would really be appreciated.

Mike