There are several factors that can contribute to a weak mobile phone signal in your area.
These can include:
The distance between your home/business and the nearest cellular mast or tower.
Interference from terrain objects such as trees, mountains, or high buildings, which can obstruct the mobile signal.
Certain building materials, such as metal, concrete, or thick external walls.
Mother Nature can also play a part, with rain, snow, thunderstorms, or wind sometimes interfering with the mobile signal.