Not a parent, but I sometimes forget to answer texts myself, so I can definetely understand your daughter. Maybe she read the text while in the middle of something, thinking she can put off replying until she's done with it, just to forget to send the reply.

If an answer is important to you, creating shorthand codes might do the trick. A simple "k", "ty" or "xoxo" could be simple ways your daughter can use to instantly respond before her mind is once again preoccupied by something else. This method is actually how I solved my own forgetfulness when it comes to texting back.