A one-word reply is not always a brush-off, but it is a signal the conversation needs new energy. Stop feeding questions that are easy to answer with one word.
Switch to playful, specific, or opinion-based messages. If they still go flat after that, match their effort instead of chasing.
Answering ok with ok kills the thread. Inject something they have to actually respond to.
Closed questions get closed answers. Ask something specific or playful that is hard to answer in one word.
If you keep getting one-word replies despite good messages, ease off. Interest shows up as effort.
It can mean they are busy, not natural texters, or losing interest. Send a more engaging message before assuming the worst, then read how they respond.
Try once or twice with better, more playful messages. If the flat replies continue, match their energy and invest your attention elsewhere.
Ask specific or opinion-based questions, add playful teasing, and share something about yourself so they have something to react to.