php文件上传奇怪的行为，偶尔上传为空

I use this script to upload files via POST from my android app to my server. 95% of the time it works ok, but sometimes the upload folder of my clients is empty. The file is sent definitely, because I get the name and phone numbers (without a file selected the app will not pass any data), but the uploaded file is not written to disk on the server. I am not an expert in php, maybe I have missed something:

My upload php script:

$file_path = "uploads/{$_POST['name']}/";

if (!file_exists("uploads/{$_POST['name']}")) {
    mkdir("uploads/{$_POST['name']}", 0777, true);  
} else {
    echo 'folder already exists!';
}


$newfile = $_POST['name'] . "_" . date('m-d_H-i-s') . '.zip';
$filename = $file_path . $newfile;

if(!file_exists($filename)) {
    if(move_uploaded_file($_FILES['zipFile']['tmp_name'], $filename)) {
    echo "success";
    }
 } else  {
    echo 'file already exists';
 }