Yes, we may get new roleplayers and that's all good.
In roleplay you have to expect a turn-over of players. Remember, It's only just for fun. People drop out all the time. Ideally you find a few people who are willing to commit to a story for the long haul, but that is always a minority.
This leads to a secondary problem. Fewer writers means fewer posts and more work for those who remain as well as potentially long breaks in between. That can slow down a thread until it dies down.
Add that to the problems of "other stuff"- people go away on vacation, take exams, etc at about the same time. They may leave the forum to do personal stuff and forget to come back. If the character is playing a pivotal role in the story- that's a problem.
And finally there are the long pauses where not much is happening in a story. It can't be all action- or that gets boring. But people get bored.
Other than that, most people prefer to be reactive rather than initiate new plot developments.
Anyway, that's a short list of why players drop out and the problems it causes.
What you need is a manager who has a sense of the story, knows where it wants to go, and can lead the players. Thus the manager's job is usually the most busy.
A new chapter usually means new characters can enter and essentially a continuation of the old characters and maybe the plot with new stuff happening.