My girlfriend won't spend time with me. Help?

Me and my girlfriend have been together for 4, going on 5, months now, and she won't pay attention. I'm taking her down to the riverside at night to hang out, kick the water, and stare at an unadulterated sky, taking her out to fancy-ish restaurants, making dinner for her at home, and she won't get off her phone. She spends almost all of our time together looking through Instagram, wish, and Amazon, yet, if I ask her to put her phone down and spend time with me, she gets mad at me and says I don't get it. I'm 19, and she turns 19 in a couple months. I just don't understand why nothing is good enough to catch her attention. Is it something I'm doing too much of? Something I'm not doing? If anything points to ulterior motives, we've had intercourse once, upon her "persuading" me. I haven't tried to force it at all.
My girlfriend won't spend time with me. Help?
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