We've been trying for over 3 years without ever becoming pregnant. It's "unexplained" after going through all NHS tests but it's obvious to me there is something going on with me because, although I have a 28 day cycle I don't ovulate until day 19 or so. Therefore I have a very short luteal phase. In the NHS's eyes, my cycle is a normal length and I do ovulate, therefore all is well 
I believe the cause of this short luteal phase will be why we can't get pregnant. So for example, poor follicle production which leads to low progesterone and a short luteal phase.
I have tried Clomid and Letrozole for several months and so have had ovulation stimulated in this way, but there was no change. Ovulation occurred at the same time and progesterone remained only just above 20 at its peak.
So my question is, if the issue is likely with your eggs and you haven't responded to ovulation induction, then is IVF worth considering? When they harvest the eggs, if they are "poor" for whatever reason then this issue still exists right?