ASP Button Does Not Work When Running From The Script?

Trying to run a script what start ASP button:

document.Form1.all["btnNewPassUpdate"].click();

It is work one time only. Next script ignore it. Where can be
problem???

A full script is as follows:
<script language="javascript" type="text/javascript">
function ChangePass(NewPass)
{
var windowsize
"center:yes;dialogHeight:230px;dialogWidth:430px;to olbar:no;menubar:no;status:no;directories:no;scrol lbars:no;location:no;resizable=no";

var retVal;
NewPass = window.showModalDialog("ChangePassword.aspx?","",windowsize);
if(NewPass !== "")
{
document.Form1.all["txbNewPass"].value = NewPass;
return document.Form1.all["btnNewPassUpdate"].click();
}
else return false;
}
</script>