I have been having similar thoughts about our DD 23months for a few months now.
DD6 was verbal very very early. (complete jaw dropping sentences at 12 months
) so when DD23mths wasn't we were sure there was something wrong.
Well, when this school year started and DD6 was away at school all day everyday again, DD then 18mths just blossomed. Like yours, not everyone can understand what she is saying all of the time, but today in the car for example, she was consistently saying 9-12 word sentences and questions, sometimes 2 or three sentences in a row!
She often also changes tense and person (he/she/her/him) properly. I think it just took some time on her own to develop her voice.
We are constantly amazed at how both our DDs develop, but especially the 2nd one since we have another one to compare to.
