HP does not want to exchange wrong cartriges

I had a voucher for HP shop online and bought my cartridges through them. The printer told me what to order and I did. When the cartridges arrived however, they were wrong.<br> <br> I phoned the HP shop and explained what happened. They were very helpful and promised to collect the wrong ones and replace them with the correct ones.<br> <br> This was over a month ago. I phone every week, and get the same helpful person making the same helpful promises.<br> <br> I want better service. I want them to come and collect the wrong ones and bring me the correct ones. And it shouldn't take months.