So, let me start with a few questions that may be obvious:
1) Do any of the receivers bind to your DX6i? If they do, we at least know it's not a bad receiver. It may be something of a dumb question, but I'd rather ask and eliminate the possibility than assuming and find out that it's never been tried.
2) When you try and bind with the DX7, there should be a way to set it to bind "Auto". This will allow it to bind to either DSMX 22 ms, or DSMX 11 ms, or DSM2 22 ms. If it's set to ONLY bind in one configuration, that may be why it's not binding. I had a similar problem with my DX6 and trying to bind to my Inductrix Pro...
I have found with my DX9 that it will not bind if it is too close to the receiver. In close proximity, a phenomenon occurs called multi pathing. This is an echo of the signal. Move the receiver at least 6 feet from the transmitter and try again. This is assuming all else is right. Good luck!
Thank you for your information mjmccarron, I have several xmitters and have tried many distances but to no avail. Sprzout, I will attempt what you suggested. The recievers bind to my DX4, Dx6e and DXi. Any other suggestions i can also try.