There's a simple explanation for this - how your baby is laid. As you're still quite early on your baby has plenty of room for spins, twists and somersaults and will constantly be shifting positions. You're most likely to feel movements and kicks when your baby has her back to yours and the hands and feet are facing outwards. When she turns to face your back you're less likely to feel kicks and wriggles.
By the time you hit about 30-32 weeks you'll be more likely to notice a routine, especially if you have a routine yourself. Until then the only time you need worry is if you go a complete 24 hour period without feeling ANY movement at all, then I'd suggest contacting your midwife for reassurance. I'm waiting for baby 4 to make her appearance (she's now 10 days over ^^ ) and have discovered babies seem to have "busy days" and "lazy days" Let's face it, everyone has a day where they really don't want to do anything - unborns are the same!
The best way to get a wriggle out of your baby is a combination of warm bath and ice cold drink. Try not to do this too often as it can set up a routine where you'll only get wriggles with that happening when ideally you want baby to be moving happily without poking her to do so.
On average most midwives tell you feeling 10 or more kicks and movements a day (a single 24 hour period) is what you're after. Less than this and I'd contact your midwife just to put your mind at ease. (Most will ask you to come into the surgery or arrange to visit you to do a quick heartbeat check and make sure everythings ok)
Hope this has helped!