The antenna signal reception frequency will be different from a different model. If you are most likely on an edge area where it can work with certain antenna, but may not work at other times. The same antenna may also perform differently around neighbors.
A TV may have a more sensitive tuner than the other, or it could be that less cable is also used, minimizing attenuation. It may just be that the antenna has shifted a little bit, so she should try and repoint it. A better antenna can help as well. It could also be her cables. There are all kinds of things that could impact them. The antenna could be misaligned from wind, the cable connections could be loose, the coax cable could be wet and/or poor quality. An amplifier might help slightly and better quality splitter. Some amplifiers also serve as splitters. A higher antenna is not always better. Try raising or lowering it to find a sweet spot.
Check tvfull.com for singal reception map.
