Cracks appear in the wall due to one or more of the following reasons:

No good material used in beam construction.
Vibrator not used in concreting.
Blocks are not laid in Header Strecher method.
Vertical joints in blocks are in one layer.
Joints are not used in blocks wall.

Due to the different cooefficiency of thermal expansion of concrete & Block/Brick, Upon the completion of every building there will be a settlement in foundation which also may cause cracks in joints.