Why would her text not get to me? or is she just not interested?

so basically, iv been texting this girl. and aranging to go on a date, and we text quite alot historically, and she usually always texts back. but recently she hasnt, she might reply and be really positive and normal, and asking me questions and things. and then i might text back saying a question or answering her question, and she just doesnt reply, thats happened a few times, or sometimes she just doesnt even reply in the first place, but then says "we havent spoken for ages" when we do speak???? whats going on, i get text message reports and it always says delivered? so whats happening, what are the possibilitis? thanks.