Some tips...
The easiest way to troubleshoot your problem would be to have another CB, or any reciever that can monitor the 27mhz CB frequency. That way you can quickly determine whats going on.
If you can't hear your own transmission then things to look for would be:
(1) faulty mic or mic connector-check the connectors, try using another mic.
(2) no transmit power output-shop repair
(3) modulation circuit faulty-shop repair
If you can hear your own transmission but someone a few blocks away cannot:
(1) transmit power may be low-should be 4watts-shop repair
(2) antenna problem- coax cable cut or not connected to RF connector on your CB
Also, as ktorian has pointed out, you might be expecting too much. It could be that there is nothing wrong at all. The fact that you can hear other people usually indicates that your anntenna system is OK.
I hope this gives you some ideas. Cheers.