Why do I lose my mobile signal in some areas of my home?

The radio waves Ihal mobile phones use are al a high frequency, around 900MHz.
High-frequency waves are more skittish than those at a lower frequency; they scatter more readily, bouncing oft surfaces and getting blocked by concrele and other materials.
Al the lower end, Radio 4 on longwave at 198KHz remains pretty consistent wherever you listen to it, while - slightly higher up the spectrum - the same applies to 5Live on medium wave.
FM around 90MHz is a notch or two higher in frequency and the radio waves become more jittery.
By the time you get to those mobile phone frequencies, the radio waves are all over Ihe shop.
A strong signal from a nearby base station gets round this, but the mobile service soon becomes patchy in areas where reception is lower.