What gives the most trouble in these radios?
Honestly? The end user usually gives me the most trouble. Really.
User expectations might be wrong. They may be expecting too much range out of the radio.
Other issues I've seen with -any- radio:
Users change channels, either accidentally or on purpose.
User turns volume down.
User forgets to charge battery.
User forgets how to use the radio.
Check the batteries, make sure they are good.
Check the antenna, make sure it's the right one, isn't damaged and is screwed on all the way.
Check the volume control, make sure it's not damaged.
Check programming.
Kind of hard to know, if you can provide some more information about what they say they are doing, that would help.