VHF radio mic problems

Hi, 

I'm having problems with the VHF signal dropping out when using lapel
mics with body packs and Audio-Technica diversity receivers ATW-1127/8.  

I'm not sure whether this could be caused by the signal being too weak,
interference of some sort, or a fault in the body packs or receivers.

Depending on where the talent is standing, the tuner LEDs often switch
back and forth quite rapidly between the A/B tuners, but sometimes it
seems that instead of switching, the receivers fail to latch on to the
signal - both LEDs go out.  I assume this is because the signal is too
weak, but I wonder whether it could be caused by external interference?
The point is that the system works fine 99.9% of the time - it's just
the occasional lost of sound for a second or two that's annoying...

The range is about 50 feet from the stage area to the receivers, which
are rack-mounted with telescopic aerials on a side-wall of the control
room, behind a double glazed metal-framed window.  The arrangement is
less than ideal, but has been forced on us by the installers.

Any suggestions?

-- 
John Legon