How many IPs do you have (in total or per server)?
As a privacy service, we maintain a very small pool of IP addresses per server, usually 3-5. We need to keep the number of IP addresses low to ensure sufficient interface crowding on the servers. Crowding is used to increase the total number of users sharing the same IP address, which combined with encryption, makes it harder for a 3rd-party to attribute an outgoing connection to an incoming one. If you are looking for a large pool of IP addresses then you may be better off with a proxy service that specializes in this type of offering.