I had that problem - with level 18 characters. I don't know solution, but I might know why. You sorcerer and your cleric are too powerful and AI don't think it's worth using spells.
EDIT: I don't knowm what level you are, maybe you started with level 4 characters, but your characters are probably too powerful right now.
Aha! That might be it. They're both level 21 demigods (I thought it would be a high level continuation of the other games, so built them powerful).
This is actually really funny - my casters are so powerful that fighting is below them. They're like "Meh - this isn't even worth me lifting a finger"
And the thing is, the AI's correct. There is no threat worth their attention. My terrifyingly aggressive Earth Genasi monk rushes off and kills everything in the blink of an eye. I barely have time to target something with my archer before everything's dead already. Meanwhile the two casters are lounging around at the back, polishing their nails and chatting about shoes.
I might actually carry on with this party, just for a laugh. I can role-play it as a party of bored gods mooching around the world for a while. They're all Genasi, so it kinda fits.
Thanks for all the replies!