Coupons non Redeemable

I was completely ****** by AirBNB. They could not return my funds from a cancelled booking so they gave me a coupon to the same value. I used the coupon before the expiry date. The booking was canceled due the company using a different card for balance of payment. Now the coupon is passed expiry. now i cannot get hold of anyone and no option on their complaints page relates to an issue like this.