Files seem to be uploaded but actually not

My iphone 3g with Discover has a problem with uploading files. If I upload multiple files, usually only the first file is uploaded, sometimes none of them is uploaded. On iphone screen, it shows "Uploading" and nothing else happens. In IE upload window, it shows files uploaded already and displayed in folder. But when refresh the folder, like I said only the first file is there, or nothing appears, files just gone.