Why is it taking me so long to get pregnant with my 3rd child?

Unknown: The reasons for not conceiving and many and varied. Based on the limited information provided, it would be best to see an obgyn to investigate possible causes with you.

I do not know: Just keep trying and it should hopefully happen soon. Mid cycle unprotected sex is the best chance for pregnancy. If it goes on for too long, you can always seek fertility evaluation. Best wishes.

Consider doctor: Depending on what your age is, if you have been trying to conceive for more than a year, then you should probably look at consulting with a fertility specialist to see if there is a medical problem. If you are over 35 years of age, then you should be seeking expert advice after 6 months of non-pregnancy.

Regular cycles yet?: If you haven't started having regular cycles since your last baby, that may be your answer. Also, you may not be ovulating if you are still breastfeeding. Otherwise, try to time intercourse to match your peak fertility time ('take charge of your fertility' by weschler is a great resourse). If you haven't gotten pregnant in 6-8 mo, talk with your doctor.