I had a town like that and it's to do with distance (in pixels). You need to build a better transmitter. In short, your transmission isn't reaching a repeater, as it relies on the strength of the transmitter not the repeater, but the repeater is reaching your receiver. I can't tell you the exact specs, but I can quote the wiki.
http://wiki.cantr.net/index.php/Category:ElectronicsTransmitters
medium radio transmitter reaches 60 pixels.
long range radio transmitter reaches 90 pixels.
enhanced range radio transmitter and vehicle radio transmitter reaches 115 pixels.
extended range radio transmitter and ship radio transmitter reaches 150 pixels.
Receivers
Everyone in the same location as a receiver in range and set to listen to the transmitter's chosen frequency hears the message. The speaker and transmitter are not identified; the message is along the lines of "you hear a radio message 'Mary had a little Lamb.'"
Repeaters
Re-broadcasts everything it receives on the frequency it's set on. Useful if communication is desired between remote locations that falls outside the range of a single radio.
Example: image:Repeater.png
Red and blue can't communicate directly, but with a repeater (white) it's possible.
medium range repeater reaches 60 pixels.
long range repeater reaches 90 pixels.
enhanced range repeater reaches 115 pixels.
extended range repeater reaches 150 pixels.