Poorly trained cashiers

I have had various incidences at different branches whereby a cashier didn't know how to do something on the system e.g process refund or make payment with a gift voucher and they just tell you it's not possible, until you call the head office or customer care to explain to them how to do it properly. Please, if the cashier doesn't know or understand something, can they please call a senior person to assist or atleast admit to saying they don't know instead of being rude at a customer and saying it's not possible.