Baby babble

Our girl has been talkative from an early age. When she was 3-4 mobths old she was "talk" nonstop and seemed to carry conversation with anyone who would participate.

Now she's 6 months old and doesn't babble at all. She squeals or grunts and blows raspberries. She puts her lips together and makes a "hmm, hm, hmmm" sound. But, that's it. You can tell by her face and eyes she knows you are talking to her and something I'd going on but she just doesn't babble or even back and forth like she used to.

Should I be concerned?