Hi C. -
that was totally us! Adam is a smart normal child every other way, but he didn't get his first bottom teeth until 15months - and it took like 2 months for them to come in fully -
then he got his molars - all at once, at 16 months -
only now at 20 months are his two middle upper teeth coming in - and we can see another coming through next to the bottom middle teeth - all other kids his age have 16 teeth!
I was worried too, and called a pediatric dentist - they said it is unusual but not unheard of - some babies don't get their first teeth until 2 years! my pediatrician said "very interesting tooth eruption pattern" and "nothing to worry about"
it only held Adam back a little, food-wise, I had to chop stuff up since he couldn't use his front teeth to bite an apple or peach or nectarine - but it really didn't matter once he got a piece into his mouth, he could gum it as if he had teeth.
don't worry! they are in there!