Hi Nicholas,
A person you are talking to on a dating site typically shows that they want to stop messaging you when it takes them a long time to respond to your messages, they are barely giving you much when they do reply and they never or rarely ask you questions.
Even if this person keeps responding to your messages, they are merely doing it to keep you around as a possible last option. They are most likely already talking to a number of other people on the dating site who have a higher priority than you do.