Radio Sounds in Wall

I am not gettibg radios anymore in the wall. When I am killing the walkers i the wall I listen the radio sounds but I did not get no one radio. All rewards are XP and tomato, but sometimes with sound of radio. Is a bug only in my game?
In 1 week I get only 12 radios. I will never play better this way. All my friends are getting more than 20 per week.