Bad service

I bought clothes at mr price Sandton city to my surprise when i check my slips they didn't put 1 jean when i called the following day they told me it isn't their problem that i didn't check my nag if clothes where there, i really don't know if that's how it works but please attend to my problem how can it not be their problem what if the cashier stole the Jean when i was not aware. As per slip the name of the person assisted me was THEMBEKA