false negative pg test

I have read and read about women who cant get a positive pregnancy test until they are late in pregnancy.They have had kids before and they know they are pregnant,but cant seem to get a positive.I have also read some women's personal stories where they got two negative blood tests and still were pregnant.Since there are two different kinds of blood pregnancy tests.One is just like taking a hpt.Are there any circle of moms that this has happened to.I would love to hear your stories since im going through something similar.