Thumbnails for converted files - Not Clear

I did by first big event in RAW on March 1, 2008. I adjusted the files wth Capture One 4 and converted them to JPEG. I was surprised when I looked at the thumbnail because they are not clear. Is this a problem that others have experience? I am afraid that my client will think the photos are low quality when she see the thumbnails.

Please let me know how to get high quality thumbnail with Capture One 4 like I got when I photographed in JPEG. I am assuming that there is something that can be done because I have never heard of this problem before.

Thanks.

Perry