If they are separate see if you can open a "not received" case for the missing one. Select "Report an item you didn't receive" on here:
https://www.ebay.co.uk/help/buying/returns-items-not-received-refunds-buyers/get-help-item-hasnt-arr...
and escalate it 3 complete working days later to get a refund.
If the tracking makes that impossible then you have to start a return on here:
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
select "Doesn't match description" and if necessary escalate it to eBay after 3 complete working days.
The seller will still have the option to send the missing items or issue a partial refund but do not close your case until they do. You won't be able to open another one. If items are to be sent back seller pays for the postage label which you will receive via ebay.
