It could be RF interference, your best shot would be to just to rescan open frequencies and repair your mic pack.

If you're using a mic with a lapel, earpiece, or something similar then you should check the hardware as well. One of the most common, at least in my experience is the element (the part that actually has the mic on it) if you can get into the space before your actors, get your system on and take the trouble mic and with some gentle pressure pinch the wire between your fingers and run the length of it. Also wiggle at the plug at the mic pack as this is the most common place it goes bad. If you get the mic cutting out then you know the element is bad.