Welcome to Healthcare magic forum,
If ur sure that u used precaution during the act and pregnancy
test negative, then its very important to find out the cause for her problem.
It may be due to anemia(her hemoglobin
may be low) or hormonal problem, or poly cystic ovarian disease
To find out the cause consult a gynecologist get complete blood tests and ultrasound
scan done, then start the proper treatment.
Hope I have answered ur question,