This is not common, but unfortunately it can happen from time to time. All tuners are different and respond differently to signal strength and quality. If your TV tuner picks up a channel that the DVR+(CM-7500) does not, this does not indicate that your DVR+(CM7500) is defective. The DVR+(CM-7500) tuning sensitivity/'Signal Rejection level' is controlled by software so the only improvements that can be made in this area are software improvements, a different DVR+(CM-7500) unit will likely provide the same inconsistency since all of our hardware is the same. If your DVR+(CM-7500) is not receiving a specific channel or the signal quality is not the same as you experienced on your television we strongly suggest trying to further improve your signal strength and quality by improving your antenna location, replacing your antenna with a model with more gain, or adding a pre-amplifier.
In many instances the DVR+(CM-7500) tuner will outperform TV tuners in certain areas and in other instances the TV tuner will outperform the DVR+(CM-7500) tuners in another area. If you continue to have problems with channel/signal reception with the DVR+(CM-7500) in your home we strongly recommend hiring a professional installer with the correct tools and signal analyzers to see what is causing the issue.
Please see the diagrams link attached below to help you understand how to connect a preamp in your antenna system.