There are a few reasons why your system may have a short transmission range:
1) Check to see if you are operating on an active TV station. This is easily accomplished by turning on the receiver (leaving the transmitter powered off) and observing the RF meter.
It should be empty. If there is activity on this meter with your transmitter powered off, this indicates that there is external interference. Follow the "Easy Setup -> Scan New List" procedure to pick a new (clean) frequency .
2) Check to ensure the antennas are properly attached to the back of the receiver (EM models only). Also, the antennas function best when in line of sight with the transmitter.
3) You may have adjusted your squelch level too high.
The SQUELCH setting helps defend against static coming through your system when the transmitters are off. Raising the SQUELCH will increase this protection while DECREASING the transmission distance. For more protection you can adjust the SQUELCH to "Medium" however the default setting is "LOW" for maximum range.
To adjust SQUELCH: Use the arrows to find SQUELCH. Hit SET and adjust Squelch to LOW (or values of between 5dB and 10dB). Hit the SET button to store this setting.