Advice needed pls. 1yr old still chokes using sippy cups

Hello,
I have been giving my 1 year old a sippy cup since she was 6 months
old. She doesn't mind it but after a few sips, she still chokes and
coughs. It doesn't discourage her but sure makes it impossible to
wean her off her milk bottles. I want her to use cups exclusively but
I can't risk it while she is not drinking well out of sippy cups for
water and diluted juice. I have tried 3 kinds of sippy cups, even
regular cups but she cannot get enough fluid this way.
She has no problem drinking out of a bottle, she has such a powerful
suck she is like a hoover, I have never seen any baby drain a bottle
like she did when she was a small baby.

I keep forgetting to ask her pediatrician about this problem. Is this
a medical problem or do some children just take longer to get used to
it?

thanks,
Idas