The students who stopped posting did not lose interest. They read the thread, saw the reply you gave someone else, and decided they had nothing worth adding.

You cannot prove that about any one student. You can check it tonight. Today's Teaching Day is about the reply, not the prompt: use their words, give one sentence of encouragement that is not a verdict, and ask a real question back that only they can answer.

The free session that walks you through three of them: https://www.harvestkernel.com/revive-the-thread/

One question for this space, and one word is a complete answer:

What is the first word of the last reply you posted on a discussion board?