T.M.
My daughter at 22 months is still picky on what sippy cup she drinks different liquids in.
We have the hard spouts that she drinks diluted juice in, one she drinks only water in and then still with milk she will only drink it from the Munchkin brand soft spout sippy cup (I tried other soft spouts and she did not want anything to do with them). That soft spout cup is what I transitioned her from bottles to cup full time, maybe you can try one of those. Don't stress she will come around.