Add-PSSnapin microsoft.sharepoint.powershell -erroraction silentlycontinue
$site=Get-SPSite "http://sp2012"
[Microsoft.SharePoint.Taxonomy.TaxonomySession]::SyncHiddenList($site)
$site.dispose()
Or
$site = Get-SPSite "http://SP2010"
$session = Get-SPTaxonomySession -Site $site
$session.DefaultKeywordsTermStore.ResyncHiddenList($site)
$site.dispose()
OR
Sharepoint timer job may be in stopped.
