A couple of possibilities... did you turn the amplifier off? There is a little button on the plate where the TV plugs into the cable, there is usually an LED to tell you if it is on or off. You need it on for the antenna, off for the cable. If that doesn't fix the problem, you may need to pull the plate and find out if they actually hooked the cable side up to the plate. There are two ports on the back of the plate, one for the antenna and one for the cable inlet.
Aaron
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