There are two theories here.
One is that she isn't verbally articulate in that her command of vocabulary and expression (fluent use of English constructions) and can't construct moderately complex sentences 'on the hoof'. She just isn't good at speaking.
The other theory is that she can't express herself because she hasn't gained the content to talk fluently about.
She has been a good debater in past campaigns in Alaska - she just hasn't grappled with the complexities of policy.
The reason can be seen in videos of her church. She's a born again Christian with certainty and faith. She doesn't need to know about things because she prays and reads the Bible and has God behind her.
So I think her extreme lack of fluency is because she's having to come to terms with policy issues she's never thought about before, while trying consciously to make sure that she doesn't contradict McCain (she probably doesn't known the full suite of his policy positions and their evolution and context).
The upshpot is she isn't ready and that her extreme faith based politics are a significant hurdle to learning.