Make sure each document is separate. In other words, do not upload a single large document containing multiple copies.
The images or scans you upload must be very clear. The official stamps need to be as clear as possible. They can be in black and white or in color. You will usually get the best quality by taking a good photo. You can crop the image if necessary, but make sure everything is as clear as possible.
If the file or photo is too large (in terms of MBs), try reducing its size. There are free tools or apps available on your phone for this purpose.
A few examples of free tools: