In many cases, the sperm needs to already be well on its way to the fallopian tube
before the egg is released. If you have sex the day of or after ovulation, the chances of conception occurring may not be quite as likely as if you had sex prior to the egg being released.
Since sperm can live up to 5 days, and the egg is typically only viable for 12-24 hours, it's easier in
most cases to get pregnant from sex before ovulation. (Not sure if that's a consolation for you or not but thought I'd through it in just in case.

)
Sperm contained in pre-ejaculate is not necessarily diminished, as I understand it, from previous ejaculation, though it can be. The problem with relying on that is it varies from man to man, and even then it can change over time.
Is it possible you ovulated late and the brownish discharge is from that?
You might want to check out Toni Weschler's
Taking Charge of Your Fertility book. It's fabulous.

Good luck!