Jet cash payment not reflecting

I paid my account last week (cash) also they were offline then but the lady said I could pay my account , I got a slip, but I lost the slip, now they are calling me and saying I didn't pay cause the payment is not reflecting. Don't know what to do, it's the second time now the other day my EFT payment wasn't reflecting. They need to do update their system or something things can't go on like this