TL'DR: I've been waiting for a refund that was approved months ago for months now, and can't seem to get my money. Any ideas?

Long version:

Several months ago, I was traveling on an LH flight booked through United. I'd used a United GPU (upgrade certificate) to upgrade to C. The upgrade had cleared, I showed in C on the website, and then the checkin agent at TXL somehow managed to bump me back into economy. A combined effort of UA 1k, UA staff at TXL, and LH staff at the lounge in MUC failed to reinstate the upgrade. Lounge staff then offered a buy-up and recommended that I submit it for reimbursement.

I did just that, sending a copy of the receipt to the LH customer service email. Two weeks later I got notice that the refund was approved and that I should get a check within 4-5 weeks.

Two months later, no check had arrived. I sent three emails to LH customer service. Finally, weeks later, I got an answer, saying that the check was returned, they were sorry and were going to send another one by courier. I responded immediately, once again confirming my address.

It's been another four weeks, I still haven't gotten my money. My emails go unanswered, and there doesn't seem to be a number I can call to speak with someone.

Any suggestions on how to proceed would be much appreciated!