I authorize a particular browser and device combination when you authorize the app, so if you use a new device, you need to go back to https://viewastwitter.com and reauthorize on your current browser.
If you have a new twitter account, following tons of people can get your account locked, so use the product on real twitter accounts and don't spam.
There is a limit for how many people you can follow every day. I think the limit is 5000 people, so don't add more than 5000 people from this website every day.