4yearold continues to wake frequently every night?

My granddaughter, living with me, has never slept through the night. why is this? any ideas? we, her mom and I have tried everything, skipping naps, napping early, to bed early, to bed late, oatmeal at bedtime, no food at bedtime. always practice a bedtime routine, including going to the bathroom. She fights going to bed every night as well, we watch her foods as well.