Great cell phone signal that allow you to make crystal clear phone calls, while enjoying high speed data are becoming more and more scarce in this day and age.  There are quite a few reasons as to why this is happening.  

Below is a list of the most popular reasons for weak cell signal:

1. Insulation, thickness of walls that are old stone walls or concrete slab or steel reinforced within any structure are usually the main reasons for unreliable cell phone signal.

2. Your location.  If your property is in an area surrounded by hills, mountains and dense forests like in farming areas and the likes that would be another explanation for your constant lack of cell phone coverage.  The same would be true if you were in a rainforest, desert, down in a valley or in a dip and so on.

3. If you are in a remote area with very little to no network tower (mast) this would justify very poor to no mobile phone coverage.
4. A wire mesh also known as a Faraday cage is often used in many buildings, and this prevents outside static magnetic fields, which means that mobile phone signal cannot pass through.  This is another typical, but mostly unknown factor to most people for signal loss.