Cannot get my refund

I’ve been trying to get my refund on flight that was changed and yet i wasn’t notified. It’s been 2 weeks trying to get my refund. I sent an email but there was no reply. Ive been calling your call centre and every time the consultant tells me a voucher will be sent to me before the day ends but that never happens. I don’t know which measure to take with this airline coz im now frustrated and i urgently need to book a flight using that voucher which is not being sent to me. This is the worst airline ever!!!