Given that you have already conceived and delivered one child, you are probably experiencing secondary sterility. Causes for sterility are identified in women one-third of the time;in men one-third of the time, and in one third of all cases both partners are responsible for sterility. Medical investigations for sterility start with the man. This is because the diagnostic procedures for evaluating a male’s fertility are much easier than for a women --- and start with a sperm analyses (spermogram). If a man's fertility is lower than normal, there are several further diagnostic procedures (physical examination, ultrasound scan, hormonal analyses, sperm and urine microbiological tests, genetic test, testicular biopsy etc.) that can be performed to identify the reason for male sterility. Unfortunately, reasons for male sterility are unknown in 50% of cases. Known reasons for male sterility include: alcohol abuse, smoking, varicocela, genital infections, hypogonadism, genetic disorders, etc.

Diagnostic procedures for females are more complex than for males. These diagnostic procedures include: a vaginal examination, Pap-test, hormonal analyses, ultrasound scan, microbiological tests for STDs, HSG, laparoscopy with hysteroscopy, genetic tests etc.
You can consult a gynecologist about this to begin to request necessary diagnostic tests.