Getting logon/logoff logs for a remote computer: not all events are being recorded

I’m using this script: https://github.com/pdxcat/Get-LogonHistory/blob/master/Get-LogonHistory.ps1

While it works, I’m getting a bunch of logon events back-to-back but no logoff events in between. So I don’t know how long User1 has used the computer for, or when User2 signed on, etc. I really need a way of getting how long each user uses the computer for. Is this a limitation of the event log or a bug in the script?