HopeFaith, i wonder about stuff like that all the time. i think it is a combination of everything - I think mostly sperm DO make it up to the entrance of the fallopian tubes. From there on luck plays a hand - can the sperm stay alive long enough to reach the egg, when they reach the egg, is the egg fertile, do the sperm have enough energy left to penetrate, when they do penetrate, is the fertilised egg genetically stable enough to survive?
And then if all of that goes OK, the fertilised egg then has to embed itself in your lining - so is the lining thick/thin enough? Even if it was genetically OK to get that far, it may still have some genetic defects which mean a full term pregnancy doesn't go ahead.
That is what I figure anyway! LOL