Nady UHF-3 wireless guitar system. Second transmitter does not work with receiver.

I purchased a new Nady UHF-3 wireless guitar system. The unit consists of an analog receiver and transmitter on 509.55MHz (MU-6 in Nady speak). The transmitter supplied with the receiver work fine. I purchased an additional transmitter, at the same time, with the same frequency as the "set". This was all purchased directly from Nady. The additional transmitter does not work with the receiver. Only one transmitter at a time is turned on. Nady set me a second transmitter and I have the same results. Nady has no answer for the problem. I have tried the volume control on the transmitter, and the squelch control on the receiver. One note, when the working transmitter is on, turning on the "non-working" transmitter will bump the working transmitter off of the receiver. This makes me think that the "non-working" transmitter is sending a signal. I don't have an O-scope or frequency analyzer. Any ideas out there? I could sure use some help! I cannot find any on-line schematics for either transmitter or receiver.