I was going to suggest a range check, too, but Tony beat me to it, lol. Definitely do a range check and see what you get?
If you are referring to the new DX6, then the technology in the two tx's should be the same. Now, the programming and the setups are different because your talking about a 6 channel tx and a 9 channel tx but the signal strengths should be very much the same.
If you do a range check and find that there is a difference in the range, definitely replace the rx and try again. Range checks are VERY OFTEN overlooked and not performed. I've been guilty of not doing a range check, too. However, this may help you determine of the problem is with the tx or the rx.
Best of luck guys!!!
Please report back with any results..good or bad.