Connection Throttling?

Hi guys! I don’t really know whom to address this message. We have a problem with our VPN channel between Boston and Chicago. In Chicago we have internet from Innovative and the speed in our VPN channel is not going above  300-350kbps. We’ve spoken to Innovative about this and they told us that  it is Comcast’s problem. They said Comcast shaped pipe between Comcast and Cogent. I don’t know why you are doing this, but this is a serious problem. We have six campuses and even here in Massachusetts we have a campus in Worchester (provider Charter) and between Worchester and Chicago everything works just fine. So here is my question: whom can we  communicate to resolve this problem?