Terrible Customer service!

I placed an order on the the app on the 14th May and received my order with 2 items missing. I called the contact centre and was told that they would follow up and get back to me. On the 19th May I still hadn't heard back so I called in again and received a reference number - *** and was told that my stuff would be delivered soon. It's now the 7th June and I still don't have my missing items, nor have I received a follow up call, email, sms or whatsapp. This is not the first time that this has happened and when it happened previously, I received a voucher to use but my app was updated automatically so that voucher went bye bye before I got to use it. I have heard so many people complaining about this service but I had yet to experience it for myself until last month. I would like someone to get back to me and either deliver my missing items or refund me.