Q: Why is the rock not sticking on the wall?

A: Remove any loose stone and mortar, clean the stone well, re-mortar it into place and redo your mortar joints. Use bonding agent in your mortar.

Q: Why is rock falling off the wall after the job is finished?

A: Stones were disturbed before they had set up.
Stones were not tapped into the mortar to create suction.
Note enough mortar was put behind the rocks or it was poor mortar.
Bonding agent was not used on your Masonry wall.
Poor rock that flakes off like mica was used (not our rock!).
Dirty rock was used.
The building settled and cracked the stonework. The footings were not adequate or settled too muck, (or perhaps a frost heave caused it).   
The metal lath was not secured well enough.
The mortar froze.

A: You did not use enough cement in your mortar.
You used bad cement.
There was dirt in your sand, or you used poor sand. The sand must be sharp and clean.
There was salt in your sand
The moisture was sucked out of your mortar too fast so it dried too quickly and did not have a chance to cure properly. Bonding agent helps to prevent this.
Mortar was used that should have been thrown out. Do not add water to drying out mortar unless you really know what you are doing. Once the chemical reaction has taken place, the mortar could be shot.
Your mortar may have frozen.

GRANITE
Granite is an important structural and decorative stone. It’s used for immense structural work due to its high compressive strength and durability. It is the hardest structural stone. Granite’s rich colors, stiffness and density, make it an ideal choice for flooring, counter tops, vanities as well as exterior applications. Many stylish patterns and colors make granite the most versatile and trendy of all stones. It is also a very low-maintenance stone. Granite is composed of quartz, feldspars, micas and other minerals. These minerals contribute to the color and texture of the granite stones. Granites get their wonderful variety of colors and patterns from minerals that are melted into the liquid mass as it is formed..

MARBLE
Marble has been valued for thousands of years for its rich palette of beautiful colors and appearance and is perfect, pretty much anywhere in the house. Marble stones consist of limestone that has undergone heat and pressure. A transformation process takes place when the weight of overlying material, pressure from crystal collisions and heat from the earth's core generate temperatures in excess of 1800ºF.Texture of marble depends on the form, size and uniformity of grains. The element components of marble determine the color of the stone. Generally calcite and dolomite marbles are of pure white color. Variations of whiteness of pure marbles are due to the mixture of foreign substances. Such impurities form streaks and clouds.