[ADTech]

First, verify that the input switch on the amp is set to "Combined" and that the FM filter is "On" or "In".

Then, look at the antenna mounting location with a critical eye, looking in particular for proximity to nearby metal objects that are horizontal or diagonal relative to the antenna in front of or near it. Look to see if your signal must pass through nearby power lines or if there are nearby power transformers as both have a tendency to put out interference that interferes with weak VHF signals (your VHF signals are very, very weak).

You've got a lot of FM stations in your city and their signals are probably 50 dB stronger than the VHF signals from DC. An external FM filter between the antenna and amp (in addition to the internal filter) may be needed. Radio Shack 15-0024, about $5, available in most stores. It's a cheap diagnostic that will either fix the problem or eliminate FM interference as a possibility.

Beyond that, look for other sources of local RFI such as electrical or electronic devices in or near the home including lighting products (compact and standard fluorescent lighting, LEDs, dimmers, solar power or other outdoor items), computers, laptops and power supplies, microwaves, solar-powered security or lighting products, plant grow lamps, aquarium heaters, timers, and even the TV set(s) themselves have been implicated in causing RFI into the VHF band.

Based on your plot, there is very little VHF signal likely to be available at your location. You will likely need to step up your game with a much higher gain VHF antenna to improve your odds. Do check the other stuff first as, if one or more of those conditions exists and is left undiagnosed and uncorrected, a larger, higher gain antenna will likely suffer the same difficulties.