The problem with updating probably has to do with caching of the file which is being downloaded. A common work-around for this problem in MsgPlus scripts is to just append a random string/number at the end of the URL each time something is retrieved from it.
eg.
code:
var randomNum = Math.random() * 100;
MsgPlus.DownloadFile(url + "?" + randomNum, MsgPlus.ScriptFilesPath + "\\DP")