It sounds like there may be something shielding the signal between the doorbell and the receiver. Usually this is the result of a large piece of metal. If you have a metal door or even an aluminum screen on the door, this could be an issue. The signal will spread out from the transmitter, but depending on the frequency, may not diffract (bounce) around objects well. Heating ducts can also interfere with signal transmission.
Test the bell and ringer in an open space to get a feel for the distance it is broadcasting. Look to repositioning the doorbell or receiver to avoid any obvious metal shields.