[No one can hear me.]

I have a radioditty GMRS radio.  Only once when I was hiking could someone hear me who was over 50 miles away.  Since that day, I hear people on channels clearly but ask for a sound check.  No one can hear me at all.  What do I need to do?  Thank you.

[GM30 GMRS radio transmission problem]

I can hear people here and there clear.  No one can seem to hear me.  Exception.  One time, on the 4th of July, I was in deep woods low in a valley, I spoke to some admin and more people.  The admin was over 75 miles away.  None of this makes sense.  I never spoke to another person again as no one can hear me.  I can't even tune both radios to the same channel and hear each other.  What is wrong and where can I learn about this?  Thank you.