Bad proxies certainly don't help and I am starting to think that if this kind of activity is carried out by many others using the same proxy address, then Twitter simply stores that as a bad IP and automatically removes anyone who creates an account on that IP address.
I realise the OP didn't use the same IP on all accounts, but what happens if the proxy used is a well known proxy and regularly used to create tens or hundreds of twitter accounts? If that's happened already, then your accounts were doomed before you even began :/
Personally, I'd spend the extra money and buy private proxies and limit each IP to a max of 5 accounts and then follow the advice found here for tweeting, links and how long to wait before blasting it.
Using RSS feeds from things like Twitter feed and locating high quality blogs for the RSS links from BBC, Sky, CNN etc and using hash tags can really help make the account look worthwhile and more like a service rather than a bot.