Sometimes the sperm doesn't quite meet the egg.
Sometimes there's an issue with the egg, and it's quality.
Sometimes it does, fertilization happens, but then implantation fails to happen.
Sometimes fertilization and implantation happens, but there is a problem with the fertilized egg (normally chromosomal), and it doesn't develop properly, and gets shed like a normal period.
Scientists say only about 50% of fertilized eggs actually implant properly.
With all these factors, a healthy fertile couple have a 20-25% chance of conceiving each cycle.