I am trying to refund a client who paid via YOCO with a credit card but to no avail

I am trying to refund a client who paid via YOCO with a credit card but to no avail. YOCO can't give me a reason why the refund does not want to go through. Gershwin from YOCO said he would return my call with an answer. He has not returned my call. It is not what I expect from YOCO.