FormData only run once when using ajax

i have a code like this

function SaveFolderThumbnail(folderInfo) {
var formData = new FormData();
if (uploadFile != null || uploadFile != undefined) {
    formData.append(uploadFile.name, uploadFile.rawFile);

    //check exist thumbnail
    if (folderInfo == undefined) {
        if (folderThumbnailSelected.ThumbnailUrl != null) {
            formData.append("ExistThumbnail", folderThumbnailSelected.ThumbnailUrl);
        }
    }
}


$.ajax({
    url: folderUploadFolderThumbnailUrl,
    type: 'POST',
    contentType: false,
    processData: false,

    cache: false,
    data: formData,
    success: function (e) {
        var thumbnailFileName = e;
        if (folderInfo != undefined) {
            SaveFolderInfo(folderInfo, thumbnailFileName);
        }
        else {
            InvokeSaveChanges(thumbnailFileName);
        }
    }
});

If this code run for the first time, it reach the ajax, pass to controller & return the filename to me, like this

        var thumbnailFileName = e; //e= "onepiece.jpg"

After that, i can save the file normally. But many time after this i select the element again, this problem show up, when i debug the code at this

var formData = new FormData();

I find out it jump straight right into ajax, but never reach the controller, which return null & the thumbnail was never been save

        var thumbnailFileName = e;// e=""

Can anybody know what cause this problem?