The reception of channels and hence their viewing in digital terrestrial depends on the level and quality of the aerial signal available at the plug the receiver is connected to. The signal will vary according to the area you live in, a correct orientation/operation of the aerial plant and the quality of signal transmitted by the digital terrestrial signal repeaters. The reason why some digital terrestrial channels are not found may depend on any of these factors.
Check the current national digital terrestrial signal cover by visiting the specific broadcaster homepages, keeping an eye out for any news concerning possible interference/work on the transmission plants.
After having ascertained the existence of a digital terrestrial signal in your area, before contacting your local dealer, if possible try your receiver at a friend's house, who already receives digital terrestrial transmissions. What's more, if necessary, contact a specialist aerial installer to check up your system.