If a girl sometimes takes 6 hours to reply?

Every once in a while this girl would take 6 hours to reply. However, usually she replies within 5 minutes when I text her but for some reason sometimes it may be 6 hours for the next reply.
So like for example I text her and then she replies in 5 mins. Then I text her back 5 mins later and she won't reply for 6 hours.
If a girl sometimes takes 6 hours to reply?
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