Mysterious adjustment layer thumbnails

[bluebird]

Hello,

I am new to the forum and have a question about the appearance of adjustment layer thumbnails or icons. At times when I create an adjustment layer the thumbnail comes in as I expect ie a levels adjustment layer shows the scale icon. But at other times the layer will show a circle that is half black and half white like the add adjustment layer icon at the bottom of the layers palette.

I can?t figure out why this happens or what triggers it to change. It has no impact on the functioning of the layers it is just annoying and I like to see the correct thumbnails so I can tell what kind of layer it is at a glance.

I?m stuck anybody know?

Thanks for your help