No help from pep about manufacturers fault

I bought school pants from pep on Friday 26/04/2019. It was the correct size on label and in the pants, but now it is too small and unfortunately I lost the slip. But this is a manufacturers fault and I feel they should help me just exchanging it not money nothing just exchange but the manager of safeway crescent refuses. Trying to phone the customer service is also really useless as no one answers. This is really pathetic from pep I'm really disappointed ?.