This isn't very common in games I play. However, I think I am the one who gets bored to character that still live, bring another in, and find out it didn't help, and real problem is this another player who constantly whines and goes against any idea of adventure.
(I got so bored of my diplomatic nice-person character at certain point).
He is also a player in one of my campaings, and oh boy, he whines. All the time. About plot, but he won't say what he would rather do. Power level, but weaker is no good, more powerful is no good and there doesn't exist any balance for him either. About his choice of character, but given a change (at any time) to change the character to different want this player just loves to whine what he should have rather done, instead of doing something about it. Ah, and this player also doesn't seem to like challenge, or lack of it, and he doesn't like to roleplay his character either. I don't actually think he likes to play rpg:s, he prefers to dm, but he whines then too. Maybe it's because he prefers 2nd edition AD&D.
There was one player who altered his character personality to become a real jerk. Well, he is not playing with us anymore. He isn't nice person in real life either.
I am guilty of changing my campaing main focus, when I get bored with it, but lack of player interest has lot to do with it too. I also tend to make encounters a bit hard. And there is this thing that some of the players (half of them) haven't bothered learning their characters abilities. Or don't bother to cast spells before the combat (and I am not speaking of surprise ones here).
In another game I played a few years back, it was very common to be left behind in some town or somewhere where rest of party were not. Amazingly it did seem to happen to everyone else except 3 players who dm liked to have as his plot focus (this was same three people whatever the campaing was).
Earlier, it was always hard to bring another character if old one died, since party was so paranoid about "new characters".
