SharePoint Search

Just to complete this issue. I've created a PowerShell script that touches all the list items in my site collections, so that search will pick up these items. Problem resolved for me! Not sure how this customer got into this situation though. 

The script can be found below: 

$cred = Get-Credential -Message "Password" -UserName admin@mytenant.onmicrosoft.com
Connect-PnPOnline -Url https://mytenant-admin.sharepoint.com -Credentials $cred
$sites = Get-PnPTenantSite | where { $_.Url -like "https://mytenant.sharepoint.com/teams*"}
foreach ( $site in $sites)
{
Connect-PnPOnline -Url $site.Url -Credentials $cred
$web = Get-PnPWeb

$list = Get-PnPList -Web $web | Where { $_.Title -eq "MyList" }
$listItem = Get-PnPListItem -List $list
$listItem.Update();
$web.Context.Load($listItem);
$web.Context.ExecuteQuery();
$listItem

}