The Banking Ombudsman Scheme says it has seen complaints about lending delays double since the borrowing rules came into effect.
The Credit Contracts and Consumer Finance Act (CCCFA) requires banks to closely vet mortgage applicants' spending habits and personal finances.
Banking Ombudsman Scheme chief executive Nicola Sladden said it had seen recent increases in complaints about lending, particularly about delays and banks not acting as expected.
Next month, the Ombudsman would release data on banking complaints from October to December 2021.