Why can´t I see my Heroku app from two computers at the same time?

Well i have developed an app that graphs some data.However I can only visualize it with one computer at a time. I use Mozzila in computer A then go to the web page and it works, then I try to go to computer B but the web page doesn´t work.. However if i try first computer B then A, the computer B works but not A.