I'm not sure, but I don't think this is a glitch. Some couples are not as able as others to have children. The best thing you can do is to keep trying (even though it's tedious). If you have enough money, you can try to adopt by going to the 'Store' and scrolling down to 'Special Upgrades', or you may just get lucky and a little orphan may knock on your door!
I have the Virtual Families 2 on the iPhone as well, but I have no trouble with my family's children.
Good Luck!