Check marks? Do you mean exclamation points or question marks?
If you mean exclamation points or question marks, this happens because you have moved or renamed the photos outside of Lightroom. You'd have to reconnect the photos inside of Lightroom following these instructions: Adobe Lightroom - Find moved or missing files and folders
If you mean check marks, then could you please show us a screen capture. Thanks
Thanks for the info. They are check marks, i will work on the link you sent.