Hi hun!
I have a 13.5month old girl and I have had the same problem. She is actually in the 5th percentile for her weight. She is 18.4lbs. Besides liquids what is she eating? My kiddo is a snacker, doesnt like a big sit down meal. She also is very picky and most times I have to force the food down her throat cuz she just wont eat. She does love cheerios, crackers, cheese, and fruits, pretty much anything she can "do herself" lol. I didnt have a problem with whole milk, she loved it from the beginning, so I can't offer some advice there.
Try offering more solid foods with lots of calories rather than focusing a ton on the milk right now. Try some mac n cheese, grilled ham and cheese sandwiches, chicken nuggets (my baby loves these and most the time will eat it) eggs, pancakes, oatmeal, anything that has lots of good calories in it.
I dont worry to much about my babies weight as she is very healthy and happy and developing just fine. It does get frustrating tho when she has her days of not wanting to eat but I just keep at it.
Feel free to send me a message if you have any questions, I know what you are going thru! Good luck and hope she starts liking her foods better and starts to put on some more weight.
*Hugs*