SharePoint storage usage does not get updated in POA Customer Control Panel for a particular site, it does not reflect actual disk space usage of a site on a SharePoint server.
You checked the Task Manager in POA, there are no failed 'Getting SharePoint storage usage info' tasks for the server where the problem SharePoint site is hosted, this means all tasks to collect disk space usage for SharePoint websites are processed successfully.
It is possible that HTTP binding is missing for the SharePoint site in question.
POA uses HTTP binding to request information about a particular site from SharePoint server. If this binding is missing, POA cannot find the site and cannot manage it.
Check if HTTP URL exists for the problem site and if not, add it.
Additional details on how to troubleshoot the issue are available below.
You can check the exact URL for the website that POA uses when sending requests to SharePoint server. One of the ways to see what exactly request is being sent from POA to SharePoint server is using MPS TraceView tool if the issue is with WSS 2.0 or WSS 3.0 site, or WPE request tracer of the issue is with SPS 2010 or newer.
Details on using these tools can be found in following KB articles:
Inside the request sent to SharePoint server when the 'Getting SharePoint storage usage info' task is being run in POA, you will see a node that looks like this included in SPSManager::getContentUsage request:
<item> <id>site_id</id> <url>**http://sharepont-site-url.tld:80**</url> </item>
So, the example above shows request for the SharePoint site with URL
Then, you can use the
stsadm.exe utility on the proper SharePoint server to check if the site can be found using binding from the request above:
STSADM.EXE -o enumsites -url http://sharepont-site-url.tld
http://sharepont-site-url.tld in the command above with the actual URL of the problem SharePoint site.
If the stsadm.exe utility cannot find the requested site, this means the HTTP binding for the site is missing. You can add an HTTP access mapping for it using SharePoint Central Administration utility. Details on how to properly do that are best checked in Microsoft documentation, for instance, for WSS 3: Manage alternate access mappings.