20 month old DD

[NotNowPlzz]

My DD is 20 months and doesn't have many words. She says
Miaow
Cat
The letter 'a' when reading her alphabet book
No
Wow
Dada

She only says Mummy when she's crying.

But the strange thing is she's said loads of stuff only once or twice, in the right context and never again...
Water
Booby
Thank you
Good girl
Cuddle
Banana

I'm wondering if I should be worried.

She also used to wave and say hi all the time. Now she won't do it at all.