If your placenta is located on the front side of your uterus (the side that faces out towards you belly) then the baby's movements are not going to be as noticeable because the placenta has no feeling. So until the baby gets bigger and stronger (and takes up more room) you might not feel regular movements.
I agree with Elyz1214. My little guy will be crazy for days on end and then nothing for a couple of days...The first time I noticed it was last weekend and I was scared to death....I posted on the 21-34 forum and got a few responses saying this was normal and the next day he went nuts again! LOL! Like Elyz said though, if you are really worried, just call your Dr. and maybe they can get you in to listen to the HB with a doppler and set your mind at ease!
there are some days that I hardly feel him move (I'm 22 weeks) and other times he moves a lot. When I think I haven't been feeling him move too much I'll drink some orange juice or eat something sweet and lay down and poke him a little (his head is near my belly button so it's easy to feel!) and then he usually starts moving around.
I see a perinatologist who told me we don't do "kick counts" until 28 weeks because movement is still so variable until then.
However - if you are concerned CALL YOUR DOCTOR! :o)