The appearance of gaps between boards is the result of internal variations of humidity within the boards. This is not related to a defective product. It is caused by environmental conditions.
There are several reasons for changes in humidity:
Boards too humid prior to installation: The boards were installed when their internal rate of humidity was too high. Increased humidity can be caused by several factors such as storing the boards in an environment that is too damp (concrete not yet cured, damp subfloor, fresh paint, etc.); boards delivered while it is raining outdoors and their transportation indoors, etc. The wood absorbs humidity and swells before installation. Once the flooring is installed and ambient humidity returns to normal, the wood loses its excess humidity and contracts, leaving visible gaps between the boards.
Relative humidity too low: The humidity rate in the air is too low (often the case in winter) and the boards tend to lose their humidity and contract. This happens quite frequently in winter and, in most cases, the boards return to their original form the following season.