Peace lillies need very good light to flower (not direct sun though). They also love humidity. They tolerate shade very well but generally won't flower in shade.

Mine is on a north windowsill and gets good light but no sun. It is potbound, only gets watered and fed when I remember or when it looks a bit sad, and it grows very robustly and flowers regularly. Perhaps you are killing it with kindness? (Or perhaps I should be run in for cruelty...)The pot sits in a larger one which will trap any water that runs through and increase the humidity around the plant while its there, but not sure that would be enough to count as a humid atmosphere.