Regenerating of previews

I have upgraded to CO 9.1.2 and have just recently uploaded 250 images. I have saved them to an album in a project within my catalogue. Everytime I open this album CO regenerates previews. When i open other albums within the same project or other projects the previews are not regenerated. This regenerating of previews slows down my workflow. Have I done something incorrectly when i imported the images from the CF card.