FLight Centre miscommunication with me and airline

I booked flight tickets with Flight Centre. Due to medical reasons I didn't fly. I sent the doctor's letter to FLight Centre and was told that Mongo airlines will not refund e, but I can use the tickets at a later date. No cvoucher was issued to me.

TOday I contacted Mango to reschedule the flight, but was told that the flight was not cancelled and they did not receive any doctor's note. They therefore cannot rebook.

I find this unacceptable as the communication from Flight Centre clearly stated that I can rebook at a later stage.

I would like this matter resolved urgently.