Sometimes when I upload an image to the Media section, to an existing image the image isn't replace with the new image. Even when uploading with a new file name and checking the "Remove file" checkbox. I usually just go into the server and remove the image itself from the media folder. This isn't ideal as most clients are uploading their own images and I want to give them a decent solution to the problem.

Can anyone suggest why this happens?

Cheers,
JV