I actually do this a lotttt, to my guy friends and girl friends, and it's nothing against them. Sometimes I'm doing something and can't text back, or I'm bored with the conversation, or I just don't feel the need to text back for whatever reason. Girls sometimes also don't text back because they're trying to play hard to get..lame, but it happens.
A lot of the time the person is either doing something else & can't text back. For me sometimes..well with my old phone I always kept it on silent ha ha so when I didn't have it near me I wouldn't know if anyone called or texted me for awhile(but its different for everyone) I doubt its anything bad. She might of just got caught up in something or fell asleep or something. don't worry
maybe they are doing something! or maybe they went somewhere, without their phone! Also maybe it's on silent, trying to play hard to get, doesn't like the subject, is bored, or just is tired, or maybe fell asleep! if she really likes you, don't worry she's just playing hard to get!