How can you be invisible on CCTV?

I have a motion sensored digital CCTV which picks up birds, cats, blowing crisp packets, children, & everything else that ends up on my property. However, we have a shared driveway which the neighbours use to walk up - and the woman who lives next door is apparently invisible!! Just the other day, she managed to get from the road, up the driveway then across to my front door, spend some time on my doorstep talking to me & then leave - all without being picked up on the monitor! She only ever appears if she's with someone else but if she's alone, she's not visible! How can this be? Any ideas?? It's truely bugging me now as it's not at all logical.