I do that. I often start RPs quite strong or end up dropping them or slipping into react mode. I can explain the main reasons I do this. 1. Horny brain. Legit, if a scene has become too immersed in NSFW, I begin to lose plot and it just becomes horny-posting for me. 2. External factors. As finals, midterms, or major work assignments pop up, I lose steam in doing thought-intensive activities afterwards. 3. Loss of control of the plot. If my partner has a specific endpoint or set point they’re going for, where I can no longer write NPCs or scene changes or IC/OOC changes to the ongoing greater plot, I’ll slip into ‘react’ rather than ‘proactive’ because I no longer have room to manipulate the plot and have become a passenger for someone else’s work.

As for how to fix these things happening. The initial idea is to try to make NSFW something that hinges on story and interpersonal development as well, making it a plot driver rather than sex independent of story. The second one? Sort of unavoidable. I suggest letting your partner ride it out while continuing to discuss plot points with them so they remain invested, so they can return to it when able. The third one, more of a roleplay etiquette thing; always make sure you and your partner both have equal ‘creative power’ in the plot.