Twitter has stricter rules now. Ever since their last update, you can't be doing the same thing as before. First of all, following 500 people with a new account is a big no no. You should start with 30-40 and increase the number by 5-10 every few days. Always wait at least 2 days before repeating the process! Another thing you need to keep an eye on is posting links on new accounts. You might get away with tweeting one or two links a day but I'd suggest you wait few weeks before doing it.