Powershell IF and not condition not working properly

I'm struggling to have two conditions met, somehow it seems only one of them works. I'm trying to do :

If a user is connected AND NOT at the lock screen, do something. The commands themselves have been verified and work individually but I must be missing something. Here is what i have:

if (Get-WmiObject –ComputerName $poste –Class Win32_ComputerSystem | Select-Object -expandproperty UserName -and -not (get-process -ComputerName $poste -name logonui)) {

do something...

}

Right now it just doesn't go in this code, it skips to the lower part where something else is happening.

What is wrong with my syntax ? Thank you for your time!