I had a think about this last night. We don’t know the whole story of every single text you exchanged with this girl, only the two she didn’t respond to. Attraction could have diminished at any point before the two individual texts we know about. Therefore we are nitpicking if we think that a single 2 sentence text would suddenly change things (which I guess it can if it came across as weak). Perhaps there are things you can look back on in the overall interaction or the date you went on with her if you really want to analyze things. Perhaps the date you went on somehow left her in some doubt about your intentions towards her. I’m not saying you did, but think about the bigger picture. But as I've found many times, sometimes there is no logical answer I can find.

Like you say girls flake for a variety of reasons so don’t beat yourself up about it, or give her earache for not responding. Move on to pastures new. Save your thoughts and energy for girls YOU think are worth it.