Getting an answer to your problem is included in your membership fee; wanting to know "how it works" will cost you extra! ?
Few facts, broad descriptions of your happening, so let me try to restate what I think you are asking, so that I can better answer your question:
You have your Wouxun KG-935G set to monitor (receive) an Amateur Radio Ham repeater in the 70 cm band with a tone set for receive (Tone A).
You could consistently hear this repeater for one week.
Suddenly, last weekend, you could no longer hear the Ham repeater, so you changed your receive tone to a different tone (Tone B ) and now you can hear this repeater again.
You asked a friend to monitor this 70 cm repeater frequency with Tone A and they report that they can hear this Ham repeater.
Possible reasons for this result (I am assuming there is no hardware problem with your 935):
1) The administrators of this repeater changed their tone to receive their repeater to Tone B; and then afterwards added back Tone A as well.
2) There are two Ham repeaters on this 70 cm frequency and your friend can only hear the one that uses Tone A and coincidentally when you listen, only the Tone B repeater is transmitting.
In any event, just eliminate any tone frequency to receive, which will allow you to receive any and all transmissions within your range on that frequency (repeater output and/or simplex) and report back your results.