You usually need a larger antenna for reliable VHF reception. A CM 4228 UHF usually works well for VHF chs 7 thru 13, plus UHF. Or buy a highband VHF antenna and combine it with the 4221, using a V/U antenna combiner.
A fluctuating signal is usually caused by multipath interference - bounced multiple signals arriving at the receiver at slightly different times. A more directional antenna may help reduce multipath, or a better tuner that ignores multipath.