Try this to see if it works.

IF you have been using a 4.8 v pack, try a 6 volt pack if you have one. The extra voltage might just be enough boost to the output of the reciever to trigger the switch. You might get another 1/3 to 1/2 volts on the output signal. At least this MIGHT let you know if it is the low voltage signal problem.

Looking at the manual posted on their site, the unit needs a signal of at least 2.7v to trigger the switch.

As a side note, hooking up a voltmeter to the signal wire to see what the voltage is will not give any meaningful results. you need a ocilloscope to see the signal pulses and their voltage.