If the webclient is letting you down, you can also use PowerCLI to expand datastores.

First, as usual, expand the volume on the storage level. Then, fire up PowerCLI, log on to the vCenter/host and run the following script, replacing the name of the datastore you wish to expand:

$name = 'Datastore1'
$datastore = Get-Datastore $name
$esxi = Get-View -Id ($Datastore.ExtensionData.Host | Select -Last 1 | Select -ExpandProperty Key) 
$datastoreSystem = Get-View -Id $esxi.ConfigManager.DatastoreSystem
$expandOptions = $datastoreSystem.QueryVmfsDatastoreExpandOptions($datastore.ExtensionData.MoRef)
$datastoreSystem.ExpandVmfsDatastore($datastore.ExtensionData.MoRef,$expandOptions.spec)