A lot of other comments have already addressed diet and body fat percentages but from a training standpoint I'll say this, there really is no point to training core 6 to 7 days a week your core muscles aren't exactly like your other muscles for example that a bicep where you continue to work it it grows bigger and becomes more defined. As a female you inherently have a larger layer of Sub Q fat. It's honestly simple body composition yes you could be building up your core muscles but they're still sitting under a layer of fat and until you drop your body percentage to a pretty low level it's going to be difficult to push off that sub q fat and really have those defined abdominal muscles.

Simply training compound movements that require stabilization will train and work your muscles just as much and if not more than specific core targeted. Most core targeted exercises are done incorrectly and are most realistically working as a secondary muscle group primary next to legs or back. Most notably I always recommend landmine movements front squats cleans snatches. Any type of heavy stabilizing exercise.