Powershell: lastLogonTimestamp blank for some users

I am developing a script to report on users who have not logged in to the domain for 6 months. I'm using lastLogonTimestamp as within 14 days is precise enough for my purposes, and I don't want to have to query each DC.

The problem is, when I run the script, the lastLogonTimestamp comes back blank for nearly 600 users. But, and here's the weird bit, I can then do Get-ADUser (which is what is used within the script to retrieve this information) for one of these users at the same prompt and it gives me a value.

What could be causing this?

UPDATE: Just to provide some clarification. Yes the DFL is 2003. Also, to clarify. I'm only referring to the lastLogonTimestamp attribute, not any other attribute.

The script begins by creating an object containing every user:

$userlist = Get-ADUser -Filter * -properties lastLogonTimestamp

Aside from some other logic, I write out each user to a text file by using a foreach ($user in $userlist) loop. Effectively

$name = $user.Name
$llts = $user.lastLogonTimestamp

When I review the text file, the lastLogonTimestamp is blank for nearly 600 users. Of some of these, I pick a test subject, for example someone who I know has logged on recently, let's say user-x. User-x will have lastLogonTimestamp blank in the text file. If, however, I then run

Get-ADUser user-x -properties lastLogonTimestamp

on the same command line after the script has completed for user-x, it returns a value for lastLogonTimestamp.

UPDATE 2: This makes no sense! I changed the script to pull the lastLogonTimestamp property in the loop rather than when I create the userlist object:

foreach ($user in $userlist)
{
$SamAccountName = $user.SamAccountName
$thisUser = Get-ADUser $SamAccountName -properties lastLogonTimeStamp

...
}

I thought this would more closely replicate what I'm using at the command line when it pulls the property successfully, but it still doesn't work. It's probably worth pointing out it's the same users every time.