Dont be dissappointed just coz the weight isn't going down, you've lost 15kg already!!! How amazing!! I started about 2wks ago, but I only lost about 1kg, but I'm still happy with myself, coz I'm happy that I do exercises every day and I'm feeling fabulous! I think it's probably the muscles been built up coz i definately can tell my tummys are more tone instead of saggy than before. So do not get upset!! It's probably also you r entering into a steady peirod of time where your body is adjusting the changes. so keep exercising you will eventually see the changes of your body, not just the weight dropping down ;-)
Good luck!