Bricklaying is a science and an art. A good bricklayer is consistantly assessing various aspects of the work as he progresses, including the vertical straightness of the wall, the consistency of the mortar, the width of the joints, and the level of each brick. If any of these factors begin to stray from optimum, problems can result, either immediately or down the road.
Mortar that has had insufficient amounts of water added will not adhere properly to the bricks. After it has dried, it will begin to flake and produce sand, and bricks will more-easily come loose. The more slowly mortar dries, the stronger it becomes, so one solution to this is to soak bricks in water before adding them to the wall. When this is done, the water will migrate out of the brick slowly, slowing down the curing of the mortar and causing the wall to end up much stronger than it would otherwise be.
Lack of Plumb
The term "plumb" refers to the verticality of a wall, something that is very important when laying bricks. A string with a weight on it, when hung down the side of the wall, should be perfectly parallel to the face of the wall. This indicates that the wall is in a direct line with gravity. Mortar and brick have very high compressive strength, but relatively low tensile strength. What this means is that they are very strong when the force of gravity and compression descends directly through them, but will break fairly easily if they are pulled apart. The more on an angle a brick wall is, the more pulling strength there is against the mortar, and the more likely it is to fail. The reason that brick arches work well is that they transform tensile pressure into compressive pressure by directing the force of gravity around the curve of the arch and down the wall.
Unstable foundations may lead to major bricklaying problems that are not the fault of the bricklayer. If the foundation on which the bricks are laid is poorly built, or is itself built on soil that is poorly compacted or excessively wet, this may lead to movement in the foundation and in the wall. Because of the lack of tensile strength in bricks and mortar, this can lead to the cracking of joints and to the eventual loss of bricks from the wall.