There may be a case when the imagery you are uploading gets interrupted. If this occurs, then open your current mission by clicking on the description title of that mission on your dashboard.
From the Mission page, click Choose Files and select the same set of images that you previously selected for your mission. Then select Upload, and allow me to upload more data.
The imagery will begin uploading and will skip the images that were already uploaded. Allow the upload process to finish and your imagery will be ready for processing.