Since you registered the copyright, you can sue the infringers for damages (which are presumed.). You should meet with an attorney that handles these matters as soon as possible.
This is a serious matter. You should speak with an attorney as soon as possible as your rights are being infringed upon and you may be owed damages. Also please note copyright law imposes statutory damages, meaning that you may not need to prove actual damages to get a judgment.
The above answer is only for information. This answer and any response does not create an attorney-client relationship between the parties and the communication is not privileged and confidential. The best course of action is to consult with a lawyer about your specific case. If you need to contact me, please call at 212-537-6936 or email at email@example.com
My colleagues are correct -- you need an attorney to evaluate your options.
Note that you really should consider pursuing some form of legal action, for inactivity once you are put on actual notice of the infringement can actually lead to a statute of limitations/estoppel by laches defense, based on your delay.