Can you elaborate on what you mean by "dropouts". To me, this word means that the signal becomes too weak and the receiver no longer hears the transmitter. In FM systems, there is often a Squelch function that determines how weak the signal can be before the audio is muted. I suspect that in your microphones, when the dropout occurs the audio simply disappears. If so, then your squelch function is working. In some systems where squelch is not used, the audio would become very noisy or be replaced by noise, so this would not sound the same as a complete dropout. The level at which the squelch function does its muting is adjustable. This raises the question of whether anyone has touched the adjustable components in the transceiver. Have you?
When you are close enough, is the audio normal or distorted?
What range do you expect to get when the system is working normally?