She only responds when she needs something from you

When someone only talks to you when they need something, that's data about how they see you. It's not ambiguous. She's not busy or shy or bad at texting. She's just not interested in having a connection with you outside of what you can do for her.

Pay attention to the pattern. How often do you reach out with nothing to ask? How often does she reply with real conversation? How quickly does she respond when she's asking versus when you're asking? The contrast tells you everything. She has time to text you about a favor within minutes. She doesn't have time to reply to your Friday night plans for three days. That's a choice, not a schedule problem.

Your reply needs to do one job: handle the request cleanly without reopening the dynamic. If you can help and want to, say yes. If you can't or don't want to, say no. Either way, keep it short and final. Don't explain what you're doing instead or why you're unavailable. Don't soften it with apologies or long excuses. Reasons invite negotiation. They make it seem like you're looking for permission to say no.

Then stop replying to that thread. You've answered the question she asked. She either accepts it or she doesn't. Anything else you add is you trying to manage how she feels about your boundary, and that's a trap.

After you reply, don't text her again for a while. Days, ideally. If she's actually interested in you as a person, she'll text you just to chat. Not because she needs something. If she doesn't, you have your answer. The silence tells you everything you need to know about whether this is worth your time.

If weeks pass and she only reaches out when she needs something, stop making yourself available. Not to punish her. Just because you don't want to be someone's on-call favor service. There are people who actually want to talk to you. Spend your time on them instead.