You could replace the existing antenna with a UHF/VHF combo antenna such as the Winegard HD7694P or any of its' larger variants (7695 through 7698).
An amplifier is not an antenna. To use an analogy, an antenna and well are similar in the way an amplifier and pump are similar. A well has to be dug deep enough to reach clean water. A high pressure pump at the bottom of a hole with a little muddy water can only deliver high pressure mud to the tap. Your CM4221HD is not designed to block real channels 2 through 13, but it's is not designed to receive them either. The CM4221HD is designed to receive real channels 14 and higher.. An antenna such as the HD769x series is designed to receive real channels 7 through 69.
I also run several HDHRs. They are the most critical tuners in the house... If they like a signal, all the other tuners in the house are happy with the signal. Do you have the HDHomeRun Config (GUI) utility installed? ("C:\Program Files\Silicondust\HDHomeRun\hdhomerun_config_gui.exe" on my PC.) It gives you the ability to monitor both the strength and quality of a signal.
http://forum.tvfool.com/showthread.php?t=820