Can she hold her head up at all?
If she's on her tummy, does she lift her head to look around. At that age i'd expect a baby to be at least able to lift her head, even if she can't hold it long.
Put her on her back, and pull her gently towards a sitting position by her hands- does she bring her head up towards you with her shoulders, or does it just hang back ? She sould be bringing it forward at least a bit by now.
If her neck is really floppy still, or it is worrying you - get her checked out, or even just mention it at her next immunizations.
Maybe you've been over carefull about always supporting her head, or she hates being on her tummy- and she hasn't had alot of opportunity to build up some muscles?
If she doesn't seem especiallly floppy in ger=neral- like she has controll of her little arms and legs, then I wouldn't think it could be anything too serious.