Complain of un adjusted credit

On the 4th October I returned items that I couldn't sell but one item was not credited till today I cannot get an explanation of why, I had one return packet for all the items and 2 picking slip as it was 2 different invoices.

I need someone to urgently contact me as this shows theft with in their department where as consultant will end up being liable for such whereas we have joined the business to make extra money but we end up being ripped apart buy the same people because of their dishonest employees.