Why won't they bathe?

I have two silkies who are lovely - they sleep in an aviary at night and have a fenced off grassy run during the day - I have made them two sand baths - one is in their preferred hanging out area and one around the side of the aviary with a plant for shade. I filled them with sand from my paddock... I stalked the bigger girls to find the preferred sandy spots and stole some from there.

The thing is, they won't use them - they stand on one and Fizzy kind of pecks in there. It has been raining a lot so the sand isn't totally dry - is that the problem? They lay about about sunbathe next to it - its driving me mad!

And don't they need to bathe for their health?