salesforce - Third application pop-up not displayed when called from a Salesforce page

Question

I developped a page in Salesforce that called an inner application with this code:

    <apex:page >
    <iframe src="http://xxxxxx frameborder="0"  scrolling="no"></iframe>
    </apex:page>

This application contain some buttons that display pop-up. When I access to the application from the Salesforce page and click on a button, nothing appears. On the other side, the alert/pop-up from the calendar, managed by Salesforce, run correctly.