Hard to say.  The LED indicates that the Rx is receiving that channel and so if its off it means that the frequency is not being picked up.  You could have a dropout or local interference that goes away when you change locations.  It's possible that your unit is intermittent (but not likely).

The good news is that simply not having the loss a frequency does not affect your audio quality at all.  It may affect your max distance a little.

Have you tried switching to other channels and does the same thing happen?  If so then you probably have some line of sight issue.  If not you have interference or a local "black hole".

Like I said ... there is no problem here.  You are experiencing a near/far issue but it in no way affects your audio.  If this were an analog system it would be causing interference and loss of signal quality and/or noise.  But because the system is all digital it does not affect the signal or it's quality.

If not seeing the light bothers you you should turn off your transmitter while your bass player's unit is on and perform a scan function as outlined in the owner's manual and then switch to an all clear channel.  I personally wouldn't bother.  It is happening in this one location because of the exact proximity and it goes away in other locations because of the exact proximity.  Cool thing is ... digital doesn't care.