With a baby who's nursing well, I'm less likely to suspect tongue issues, but assuming it's not a hearing issue, my next guess would be a physical obstacle to producing certain sound.

My DD was not a talker or a signer, so got flagged for possible expressive language delay. She drew some amazing-for-a-22-month-old stuff at your DD's age, and then drew nothing identifiable for the next two years. And she's turned out fine, if perfectionistic. So there's not necessarily anything to worry about.