@Edent oh, antennas and antenna placement can be very weird to understand and to debug. in general it's better to not have your whip antenna near to a big piece of metal because it will couple with it and throw everything off.
recommendation: take a photograph, measure for a day, move something around, take another photograph, do another day's measurements, repeat.
If you can get the antenna outside rather than inside it will make a difference, but that's not always easy