I've noticed this many times. Back when she was the cheerful, yet vapid, vixen, flirting through the show with Joe, she seemed fine at small talk. This all changed, IMO, at the time they cemented their relationship and she took on the role of an "equal" with him...she lost her carefree persona and became serious and ever more tongue tied. She seems to worry constantly about what she will say next and thus can't follow the conversation spontaneously. She isn't a natural at this and, in a way, I feel sorry for her that she has found herself struggling to keep up. And, when I think about it, I remember seeing a clip of her at one of the shaminars where even reading from a script there, in front of her people, she kept messing up. A great conversationalist she's not. Joe obviously takes advantage of this.