I don't think you should have a problem with that length although you can always increase the signal by turning the small screw in the "+" direction on whatever port you're connected to. I think it's default to half.
Have you tried using an emitter and holding it in front of the unit?
Do you have a visual feedback emitter to see if the signal is in fact reaching the receiver?
Did you make sure that the ring is connected to the ring and the tip is connected to the tip on the ir cable (3.5mm) Reversing it will cause it to fail.
Did you use the adapters that come with the RP6?
Just some ideas you might what to try or double check.