I have found that for me, when I'm feeling that way, that I'm in a rut. So I plan something fun and different - beach day, water play, an outing whatever. Just something that I'm looking forward to. And then I work really hard to enjoy it.
And yeah, the mix of kids can really matter to. Some kids are a bad fit for some houses and caregiving styles and may need to be asked to move somewhere that would be a better fit for them. I especially see this with the older kids - they really need to spend some time with their peers and if I can't arrange that (and I can't always) they can become really unhappy and awful to deal with. I've suggested a move to the local Boys and Girls club for a few of them.