I had an anterior placenta with DD and didn't really feel her 'till around 24 weeks. I never felt her frequently even after that. It was crazy how little I felt her. DS after her was a posterior placenta and it was just amazing how much I DID feel him. With DD I'd say I only felt about 20-25% of all her movement all the way to the end.
ETA: another thing that stinks a bit with an AP is that the 3D u/s where you can 'see' the baby doesn't work through placenta. Sometimes they can see around the placenta by kinda aiming from the side. It will depend on just where your AP is. MFM explained that for a regular u/s the waves go kinda straight through but with the 3D kind it's more like a net the way the waves go so you'll see the placenta not the baby since it's in between. :-(