Sorry to hear a part is missing. It doesn't happen very often; we hope you understand that models with so many small parts that this type of thing can occur occasionally due to the nature of a model having so many parts.
As we are a Cobi retailer, we only have complete sets, we are unable to stock spare parts for all the sets. So, the procedure that we and all Cobi retailers follows is to direct our customers to the Cobi website. There is a form where you must enter the Cobi model and the part number. They also require you take a photo of the Cobi manual highlighting the missing part.
Cobi will then mail the missing part directly to you.
Once you have completed the form you will get an automated email to acknowledge your request and then it is in Cobi's hands to get that part to you.
Let us know if you have any problems.