The first thing you have to do is see your gynecologist to rule any physical problems, whether from your past surgeries, or a new problem. It may be that scar tissue has developed. The doctor can check to make sure you are ovulating and work with you to find a cause.
Health Central has several articles about this:
The American Pregnancy Association also has advice about Infertility.
I'm sure you have done a lot of reading. The place to start is at your doctor's.
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