I had my students in groups. Each group submitted a PDF to me at the end of the lesson. When I went to the web version of Speedgrader, some of the submission showed as submitted but I saw "No files were uploaded for this submission." When the students insisted that they had turned in the PDF, I went to my Speedgrader app on the iPad and sure enough, the PDF was showing there. This only happened to about half of the groups. What is going on?