Bear with me, its a longwinded one!!

I am trying to create a new dynamic distribution group with a custom filter using Powershell commands

I have created the following group

new-dynamicdistributiongroup -name "HE Students" -includedrecipients mailboxusers -organizationalunit "walcat.ac.uk/E-Mail Groups"

I then set the filter to pick up only students with certain words in the "Description" attribute of there AD accounts.

set-dynamicdistributiongroup -identity "HE Students" -notes "HE Students Only" -recipientfilter {Description -eq "*HND*" -or Description -eq "*Higher*"} -recipientcontainer "student.walcat.ac.uk/student accounts"

This all works, or at least appears to, there are no nasty red error messages.

I now try to list the members of these groups

$HESTUDENTS = Get-DynamicDistributionGroup -identity "HE Students"
get-recipient -recipientpreviewfilter $HESTUDENTS.recipientfilter

Nothing is displayed, its like the group is completely empty.

Can anyone help? Am I doing something wrong?

Thanks
Mike