If the engineer visits a site for the 2nd time and it is the same Call Number, then the PO will not automatically generate.
The option to automatically generate a PO is triggered when the call is despatched to the engineer.
Alternatives would be to:
Undespatch and Redespatch the call to the engineer
Have the engineer close the call, and have the service desk raise another call and despatch it to the engineer
If you have the option when closing a call to mark it as Another Visit Required, you can also use this and then tick the box named Assign & Dispatch to me.