There's in the trigger a "Countdown" place. Make sure it's pointed to that, no "timeout". On the rigth side you can see min max and mid boxes. Write on everyone of them the time in seconds you want it to wait. And this goes into the trigger where radio message is.
Like: min 2 max 2 mid 2
If you have a outer file script in this mission and there are your radio messages; write there like ~2, or what you want it to wate. So this is beteween the radio messages.