there are a number of settings in PHP that can effect file uploading, you have a number of file fields on your form, so it could be files size, or it could also be the time it takes for the form to submit, or the size of the form data. see the following page for details on settings that effect file uploads:
for each file upload behavior on your page, there is a binding collection named WA UPloaded Files (File Field Name). you can use the Status binding to show an error if file upload failed.
see the "Customizing Error Messages" tutorial on the Universal Email support page for details: