The network is working on a set of model codes of trustee conduct in a bid to address the issue.
"If you put a group of people together, they don't magically work together," said Claire Farmer, head of operations at CTN. "We all know that, yet somehow we don't expect it on charity boards because we assume they are all nice people."
She said that boards faced other common problems, such as trustees being too passive or failing to respect confidentiality.
Farmer said that most large charities already had codes of conduct but rarely used them, either because they were too generic or because boards lacked the confidence to confront trustees with them. "When you are dealing with volunteers, you need them to stay, so you try to be nice to them," she said.
CTN plans to produce two or three model codes to suit different kinds of charities. "We don't want to say 'here's one code for the whole sector' because that wouldn't work," she said.
The charity is launching a consultation project, partly funded by a grant from the Joseph Rowntree Foundation, to help it develop the codes. It is calling on trustees with experience of codes of conduct to get in touch and has also posted a questionnaire on its website.
It hopes that the codes and instructions on how to use them will be available by November.