Why didn't Majel Barrett have a role in the Star Trek film sequels?

She appeared and had a significant role in Star Trek: The Motion Picture and had a cameo in The Voyage Home, but that's it. I find it strange she wasn't in The Wrath of Khan in the first place or any of the subsequent movies excluding Star Trek IV. After they kicked Gene of the films, did she refuse to participate in some sort of solidarity with her husband? Or did the makers of the film just not think her character was significant enough? I would find the latter odd since she was in all three seasons of Start Trek, a character in the Animated Series, and had a substantial role in the first film. One could argue her position was redundant since they already had Dr. McCoy, but script writers could easily work around that.