"?" Icon on thumbnails

COP thinks files are missing as I get the "?" icon on my thumbnails.
The files are there and haven't been moved but in the folder section I did notice it shows no images so I re-synchronized that particular folder. Still have the "?". I added the files again in my album now I have duplicates. I tried the "locate" function too I thought this would be like LR but doesn't do anything when I locate the file.
Is this a bug?