Quantity surveyor querie

I am a main contractor, every month I submit my valuation sheet.
The qs adds to this what he sees fit and deducts exactly the same. He has put my ohp on the electrical works spread over the total job cost, no other ohp has been done the same as this.
He has made several mistakes or overpayments to us but notifies us of this after our valuation sheet is submitted leaving us with less money to progress.
We are working under contract (JCT) is this norm
al or even allowed?
Thanks