Refusing to refund

On the 19th of April Momentum debited an amount of R2294.50, I then called them and requested to cancel the car insurance policy as I am already insured with another service provider. On the 21st of April a consultant from Momentum called to advise that I will not get a full refund as the assumption is that I had cover from the 1st of April until the cancellation date. The correspondence from Momentum states that after 2 failed premiums the cover will be cancelled, I've had two failed debit orders, why are you still debitting my account?