lol i think I've been on both ends of this problem.
I have one roleplay. And the majority of the people on my friends list are friended through that one roleplay. So I assume they have the general knowledge of that. Yet I often have people whispering me while I'm in said roleplay map asking me to join their roleplay because theyre lacking members, or because they are just bored.
Like you, I like the friends I have (most of the time) but it gets to a point where they don't understand that I'm only able to focus on one thing at a time.
On the other hand, I'm also the annoying roleplay leader who, if the group is roleplaying, I whisper you giving you a heads up. I've gotten a variety of responses throughout the times doing this, and its varied from "I'll be right there!", "I'm busy with another roleplay, but I'll head over right after!", to "Yeah... I'm kinda busy right now, so maybe some other time."
More often than not though, these users dont bother showing up at all, and I can understand that lol. However with one "roleplay" day a week, it wouldn't be too hard to just pop in for a bit. It's not a big commitmen-/shot for ranting