After installation you need to test the coverage of the transmitter in the room itself.
One option for the lack of reception is that you are testing an area that is not covered by the footprint of the radiator. Another reason is the presence of objects between the radiator and the receiver that block the infrared radiation. This could be structural elements such as balcony, poles or beams.
When the infrared system has multiple radiators which use different cable lengths, the signal will reach the radiators at different times. As a consequence the radiation from the different radiators can cancel each other out. Solve this by using the automatic delay line compensation (). It is possible that you may need to tune these settings a little inside the room.
Typically you can identify a black spot as an area of bad reception that only occurs along a specific line and/or rotating the receiver into another direction will restore the reception. Confirm this type of interference by turning of one of the radiators. Typically higher bands are more sensitive to black spots.
Other systems using infrared at certain frequencies can disturb the reception of the receivers. Disable the bands that can experience interference using the web server. See for more information.