Are you only using email to promote yourself and the web?
If you haven't considered to mail your compcard and headshot to agents then you might not really book serious work as a model.
To get work with brands that everyone knows world wide it takes working with the right people, the right agents and going to the right castings. Getting the opportunity doesn't come from simply using the web.
You need printed material of your self, printed photo and this is also called a comp card, an example is enclosed. A compcard shows your diversity and it is your best marketing tool.
Mailing physically the agents your compcards is the best way to get results.
It is also best to stay busy when you are submitting and to not give up! Just because you don't hear back doesn't mean you shouldn't model, maybe it is your photos, maybe it is the timing, maybe the agency isn't interested now but maybe you should mail them in another few months and try again!
Print is the best way to get the attention of an agent. Agencies are used to mail and getting it directly at their office.
I would not stop by the agency without calling and asking if they have open calls first. Open calls let aspiring models, show their portfolios and get a chance to be seen by an agent. Anyone can go to an open call, staying inside the agency and getting on their board is a different story sometimes.
It does help to have some experience, have had a few photoshoots, and even booked a modeling job yourself. Perhaps with a local magazine, or aspiring jewelry designer, anything that can say "I have the ambitious personality and drive to be a model" is a bonus!
Try mailing your photos directly to agencies if you are not having luck through using the internet.