Workflow does not start when items are added by Powershell script

I have Powershell script that imports data from csv file. It populates a list and does the job OK. The problem is that the workflow that is configured to run on item creation (and modification) does not run. If I add a new item manually WF runs OK. The section from the script that add the items is shown below:  

foreach ($row in $contents)
{

$item = $list.items.add() 
 
if ($row.Column1 -ne $null)

{
 $item["Column1"] = $row.Column1
}

$item.update()

}

Could someone please shed some light? Thank you!