Sarak,
It isn't at all uncommon to not feel movements at 20 weeks.

An anterior placenta as the other women have said will cushion the movements of the baby and this does often delay the feeling.
I have had anterior placentas with a number of my pregnancies and it has always resulted in a delay in feeling movement.
If you are truly worried - it would definitely be a good thing to pop up to your care provider - they will put a doppler on your belly and I am sure you will hear the reassuring sound of a heart beat.