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One of the common causes of this is the image size. There is a upload limit of 5Mb per image for bronze and 100 Mb for Silver.
It's worthwhile just checking your image size and if it is over-sized use an image manipulation program to reduce or compress the files size to say 20 Kb for a postcard size image. This won't affect the canvas size of the image at all and shouldn't show any loss in definition to the viewer..
If this isn't the cause please get back to us here.