I use Set field instead of an Insert step such as this.
"Insert" steps require that the field be physically present on the current layout ("invoices") or they silently fail so that would be the first thing to check. So it could be that the field is no longer present on the Invoices layout.
Set field will modify the field even if the layout is not on the current layout--provided the layout's table reference provides functional access to the field's table.
Your a star, I couldn't work out for the life of me why it wasn't working.