All my kids have spoken unclearly at different points, completely separate from their very large vocabularies and complex sentence structure and almost perfectly appopriate use of adverbs (which, as a writer, I love!). None of them had any issues with the physicality of speaking either. The just didn't say all the sounds clearly.
My oldest DS didn't use consonants much, which meant that bat sounded like hat sounded like cat. Luckily, we never had a ton of trouble understanding him because of context. Dear boy never really got frustrated if we didn't understand, either -- he would just show us or get whatever he needed for himself. Anyway, the thing that ended up helping him speak clearly was when we started focusing on phonics. In particular he liked things like "Between the Lions" on t.v. and "Hop on Pop." [BTW: We didn't do it for reading, but specifically because we thought it would help with pronunciation. He already happened to be interested in BTL and we already owned and read HOP. We just decided to use those as a jumping-off point.] We talked about how important all the sounds were in a word and that if you changed just one of the sounds it changed the whole meaning of the word. It took a little time, and it was never anything but casual and occasional, but within a month or two he really had an explosion of sorts and he started talking much more clearly. In 1st grade he had to work on saying Ls, but that cleared up quickly when he focused on it.
DD spoke the clearest, but still at 8 says Rs more like Ws. She likes to say that she sounds British. We point it out to her whenever it makes it hard to understand her, but in this case she truly can't make it completely clear. I kinda think it will come with time, but if she's 10 and still doing it we may have to figure something out.
Younger DS6 has the muddiest language. For him it's not that he can't say the words properly (although Rs are tricky for him, too), it's that he talks and reads quickly and is lazy about saying the sounds clearly. But ask him to say anything the correct way when you model it and he can. I'm not sure what exactly it is going to take to move the way he *can* say words to the way he consistently *does* say words, but right now we tend to practice when he's reading to me. I try not to correct his pronunciation too often since I want him to enjoy reading as much as he already does, but there are times when I'll point out the correct pronunciation. The other thing we do is, after a reading session, I'll have him repeat words to me that he's said unclearly while he was reading to me. It seems to help him be more aware of pronunciation.
Sorry I don't have any good answers. 19 months is still very young, though, and I don't think that any speech pathologist in the world would expect your DD to speak clearly. At that age, red flags are pretty much only raised when a child is not speaking at all or when there is a physical issue like sucking problems. Of course, that doesn't mean that both you and she can't get frustrated!
Oh, one other thing that worked when I couldn't understand something my kids were saying: ask them if they can say it in a different way. With a larger vocabulary they were often able to say it in a different way, and then when I understood, I was able to figure out what they had been trying to say. In that case, I always made a point to validate what they had been trying to tell me by saying something like, "Ah, you were trying to tell me. . . ."
