Adding a new item in Parent List not shown in lookdown in child list with lookup columns.

Hi, 

 I have set-up  a document library for which several columns retrieve their data from a parent list, making the document library the child list. This without any troubles. 

However, when adding a new item to the parent list, this item is not automatically shown in the child list. 
For example: The parent list is an overview of the company fleet , with all cars and their license plates. 

The child list is an overview ( document library in which the receipts from the carwash are uploaded ) of when the cars went to the carwash. 

Normally when i enter the license plate in the child list, all the other info gets filled up automatically from the parent list.  Unfortunately after adding the new cars in the parent list with the function ' add an item' , i am not able to select these new added items in my child list. It is not updated. 

Sounds familiar to anybody / somebody knows a workaround?

Many, many thanks in advance.