Firms supplying supermarkets are still reluctant to take a complaint to the sector's adjudicator, mainly because they fear it would damage their relationship, it has been revealed.
Christine Tacon said she was disappointed that the number of suppliers willing to report an issue remained "stubbornly" on around half.
This is despite publicity around a high profile investigation against Tesco, which found that the supermarket giant had breached a code of practice.
"In the coming year I will be redoubling my efforts to overcome this fear factor and also to reach suppliers overseas, where knowledge of the Groceries Code Adjudicator remains low," she said.
Ahead of the GCA's annual conference in London, Ms Tacon said the Tesco case had shown she could carry out a complex investigation with "significant" findings which benefited suppliers, with no identities revealed.
A survey of almost 1,000 suppliers found that the top 10 supermarkets had all acted on issues raised by the adjudicator, although six out of 10 had still been subjected to an issue.
Suppliers rated Aldi, Sainsbury's and Lidl as the top three supermarkets for complying with the code, while the biggest improvement was said to be at Tesco, with most suppliers saying its practices had got better.
Suppliers had raised complaints about incorrect deductions from invoices, excessive charges for artwork and penalty charges for inaccurate forecasting.
Ms Tacon announced increases on the levy on supermarkets to fund her office, to increase the total from £1 million to £2 million.
The Tesco investigation had cost the GCA £1 million, she revealed.
A Tesco spokesman said: "It's very encouraging to read the feedback from our supplier partners and to see the changes that we've made to the way we operate being recognised.
"We know there is still more to do, but by working more closely together with our suppliers, we will continue to focus on the things that are important to our customers."