Bad service from staff member

I did a layby on 12th March and last Saturday I was with my Son at the store, realized after fitting him a jacket that the one on layby is small. So I requested to change the item, but was told that I need the layby slip to do that, unfortunately I didn't have it with me. Today I came back to the store with my slip, I'm told it's impossible to do it. Would I really have came back if I was told on Saturday that it's not possible, not really, but I came because one of your staff said I need the slip for her to do that. Today it's another person plus she called the Manager Charity who also said they can't help me because though the prices are the same, the codes are not. Please advice because this layby expires in June and I'm afraid by that time when I collect the layby, the size of that item might no lager be available . I'm really annoyed that on Saturday I was told to bring my slip to be assisted n now today no one is assisting me. I even asked if they can't scan the other size and attach that slip to the old one just as they normally do when one pays the layby monthly, but they refused.