Simple reason for poor performance ... no ground plane.
It works great attached to a vehicle because the vehicle itself is the ground plane.
What is a ground plane? It is a flat surface of ferrous metal (galvanized, steel, etc) that a magnet mount antenna uses as an integrated part of its performance. Without this metal surface the signal reception will be sub-par. This has nothing to do about reflecting the signal to the antenna. It is about the design and construction.
As long as the piece of metal it sits on is big enough you will have an adequate ground plane. Use the following dimensions:
For Dual Band/800 MHz = 14" x 14" Minimum (28" x 28" recommended)
Any cheap piece of metal will work, roof flashing, pizza pan, cookie sheet, refrigerator or file cabinet :-) ...as long as its big enough. Just make sure you mount it in the center of a circular pattern.
Remember, this needs to be a ferrous (magnetic) piece of metal. Not aluminum.