It took a really long time for me too. Even when I was 20wks-24wks I still looked pretty normal to me and had the same B shape belly I had pre pregnancy. When I told everyone at work I was 6months, no one believed me because I looked the same. I just now noticed this past week some growth spurts in my body. It hurts so bad when I laugh, sneeze, stretch so I think I must be just starting to grow more.
Just this last week, 25-26wks, My uterus has came over my belly button about 2inches finally and I notice now is starting to push outwards.
I know its different for everyone but once your uterus rises more and pushes outwards, you will start to show more and more. You're still in the early start of growth..Some women are lucky and get the bumps very early but from what I read online it generally occurs between months 5-7 for major growth.