Why do humans have a harder time conceiving then most mammels?

Compared to most species out there humanity seems to have a hard time conceiving. Even if a couple perfectly times donation with ovulation and does everything else exactly they still have at best a 25% chance of success, while many other species seem to be able to always conceive on the first try. I'm trying to figure out why it's harder for us humans.

Specifically if you time it right and egg and semen are introduced at the correct time, why doesn't conception occur? does the sperm not find the egg? does it reach the egg but fail to enter it? Does a fertilize egg fail to implant? What is the cause of the failure?

In a related note if you have to people with 'normal' fertility who is the major deciding factor in pregnancy? is the most important factor the quantity of sperm, quality of sperm, or females body/eggs?

Thanks for anyone that can either answer the question directly or link to something that provides the answer. It's proven surprisingly difficult for me to Google the information.