J.F.
My daughter did this, too, she wouldn't tilt it. You just have to hold it until she figures it out, which she will, probably overnight, ha. Sometimes I would hold the bottle and put it down, then back up, and down again, as a sort of lesson, in hopes that she would figure out that 'down' means no milk and 'up' means milk. I also tried the switching back and forth, not a good thing. Stick with one thing, she may get mad but if she's hungry enough she'll take it one way or the other.
Word of caution, when my daughter started teething the soft spouts were good for gums, but when she had all her teeth come through she chewed on the bottle and sippy cup spouts until they tore, causing milk to spill everywhere. If you want to switch to sippy cups permanently, try the hard spouts.