First, the saying might fit, you can bleed BUT NOT ovulate, but if you ovulate you are going to bleed. How do you know the cycles of 21, 17 and 24 were ovulatory. Seems a little erradic and short to be ovulatory cycles. Did you do a sonogram, check an LH or Progesterone. I would work on securing that you are ovulating, the HSG test to check your tubes isn't alot of fun or cheap.
Hope this helps,
JM