Image styles do never or seldom apply

I have a content type with an image.
I made 2 styles: float left & float right, mostly applied to images, but also to Share buttons etc.

When I apply a style to a normal field, all goes well.
But when I apply the style to the image field, almost never that style is applied: the class does not appear in the HTML output, even when clearing cache.
When it rarely does apply & I want to change it later, changes are not applied and I'm stuck with the old style.

I see this behavior in ds 1.2 and the current dev version.