Initial steps to isolate sound problem

Please can you suggest the first steps to take to solve a singe channel sound problem?
Such as how to isolate the cause of an echo, delay, reverb when one person is speaking from a lapel mic.
What happened this morning was with other mic inputs muted the only mic in use gave the impression of being in an echo chamber.

I realise I did not give details of the system we are using. It is a Qu-SB (Allen & Heath QU-SB Compact Wireless Digital Mixer 32 Ch ) using Qu Pad on an iPad connected via Apple Airport Express Base station.

Select the problem channel. Then push the FX1 or 2 buttons. See if that same channel fader is turned up any. If so that’s your problem. That determines how much of that mic goes to the FX. Then go back to LR mix for normal operation.

As mentioned you may accidentally have the lapel mic channel
FX sends turned or your description could also kind of describe what a
lapel mic turned up in large room can sound like if not properly set
up and EQ’d!

Thank you for these replies, we will look into and try your suggestions when we use the system again next Sunday. It is a large hall that we use and often the person using the lapel mic has a soft voice.

A large hall and a soft spoken person on a lapel mic is not a good combination!!