Good question! Large telephoto lenses seem to have a tendency to “disappoint” users in certain circumstances. I’ve actually been doing some research recently on 400mm lenses, so I’ve seen lot’s of photos taken at 400mm which were anything but stellar.
My best guess would be autofocus issues. Seeing as it’s one of Canon’s prestigious “L” series, it definitely shouldn’t have any sharpness problems! Were the pictures taken in low-light conditions by any chance?
That said, without being able to see what “problems” you are seeing, it’s rather hard to diagnose the problems. If you don’t mind, could you post a few samples? Ideally, I’d like to see the originals, at full resolution, (it’s very hard, if not impossible to judge sharpness on a resized/resampled photo). If they are RAW files, and you use Lightroom, then you can export at lower JPG quality (77) to save on bandwidth.
Please note that this forum only supports files up to 2MB in size, so an original photo file likely will be too large. Probably your best option is to use another service such as Dropbox or Google Drive, or even Flickr, and then post a link to them here on the forum.