It seems first time you set the default page . but when you reupload it again it again overeright the default page related configuration.
Please do following changes in your web.config.
<system.webserver>
<defaultdocument>
<files>
<clear />
<add value="Your page Name here" />
</files>
</defaultdocument>
</system.webserver>
Hope above solution will help you !!