Since this is roleplay, it's almost impossible to make sure a detailed plot can be followed entirely. That's why I instead identify an end-point, and don't really fuss too much about the journey itself; so long as it's an enjoyable one, it all works.

While writing each post, I keep an eye on my end-point and try to ensure I keep moving towards that. If my writing partner doesn't play ball and moves in a different direction, it's fine to let go and accompany them to their end-point instead.

Ideally, we can tread the same road and both get some character development and progress in, so whenever we thread, I try to figure out what my partner wants to get out of the thread.

Since often, roleplayers are pretty laissez-faire and want to follow where the winds blow, it's often plenty to gently steer in the direction you want, rather than going along with whatever happens and hoping neither of you walk into a dead-end.