It could just be the way your baby is positioned inside you (towards your back) or you may not be showing as much depending on your weight. As nutz said, some people don't show at all (think of all those women who didn't know they were pregnant at all ...they probably didn't really show either).
If you're worried that your tummy's not growing because your baby is not the right gestational age, talk to your doctor about an ultrasound to check the baby's age. Also speak to him/her about not showing.
I, too, thought that I just wasn't showing like I thought I should be. But after looking at pictures of myself, I've realized just how big my tummy's actually gotten. Try taking a few tummy shots (if you haven't already) and compare them to future pictures.
hey hunni try not to worry some people dont show at all, others dont show till late in the pregnancy and others show staright away. dont worry, but if you are speak to your doc or m/w. hugs and kisses.