Hi Steve,
I'm not an RF expert, but here are a few basics: do you have the aerials fitted to the receiver and have it positioned with clear line of sight to the transmitter? If not, make it so.
Are you testing it with the transmitter right beside the receiver? If so, try moving away - being too close can swamp the receiver with too much signal to get good audio (though in that case I would expect to still see activity on the receiver's AF meter at least).
I'm not sure if this applies to that model of mic but check the manual to see if there's a feature referred to as "pilot tone".
If there is, check and make sure it hasn't been turned off by mistake.
HTH,
David.