1. Problem in Ovulation
This is caused due to PCOD and other reasons.
2. Problems in the fallopian tube
This happens in patients because of issues in the uterus.
3. Because of Antibodies
Issues in the antibodies which result in issues in eggs or sperms.
The incapability of the sperm to stay alive till penetration in the cervical mucus.
5. Other problems
There may be other problems or medical conditions as well which lead to fertility issues.