PowerShell Script does not find all Document Libraries

Hello,

I have this PowerShell script that loops through my web application and reports document libraries in sites. However, most are being reported, but some do not. Can someobody explain why some are not being picked up by the following script:

foreach ($web in (Get-SPSite -Limit All | Get-SPWeb -Limit All))

     {
        Write-Host “Processing Web: $($web.Url)…”
 
        foreach ($list in ($web.Lists | ? {$_ -is [Microsoft.SharePoint.SPDocumentLibrary]}))
        {
            Write-Host “`tProcessing List: $($list.RootFolder.ServerRelativeUrl)…”
         }
       $web.Dispose();
    }

Paul