Seeking Insight from Recruiters: What’s Going On?

I’m looking for some clarity on the job application process from the perspective of recruiters in the UK.

I recently applied for a position I’m qualified for, found on LinkedIn, and reached out to the hiring manager. After some communication, we agreed that I would be a strong candidate, and he assured me I’d be included in the interview pool.

Now, 3-4 weeks later, I’ve heard nothing about interviews, and I’ve noticed that the job has been reposted. What does this mean for my chances? Does it suggest I was considered but not quite the right fit? Do recruiters prefer to have a certain number of candidates before initiating interviews? Also, when would be an appropriate time to send a follow-up message without being bothersome?