Maybe she’s just not a texter? I have a best friend from childhood that spent every day, all day with me from 4th grade through high school graduation. She went away to college and all of a sudden the communication started to break down- I thought something was wrong, but later found out she just doesn’t like texting or FB chatting and so that’s a difficult way for us to connect. It had never come up when we were in school. We talked it out and she knows how important it is for me to hear from her regularly, so we make a date to call once a month. It’s not exactly the relationship either of us dreamed of, but it’s what we can do right now and I’d rather have this than nothing.
Similarly, my own brother hardly ever returns texts or phone calls. He just doesn’t like connecting that way. Everything is perfect in person and we are as close as ever, but it definitely requires a lot of effort to keep that connection (I have to accept less communication than I want, he has to try hard to communicate occasionally). Sometime I ambush him by calling my sister, and asking her to put him on the phone for a while- so you could try that?