The move follows concerns that organisations define complaints in different ways, leading to inconsistency in figures reported to the FRSB.
The current definition of a complaint, agreed after consultation two years ago, is "a potential breach of the Institute of Fundraising codes of practice or the Fundraising Promise".
But some charities found the definition too vague when they filled in annual returns, which fed into the FRSB's first annual return this month (Third Sector Online, 8 May).
"One example is the Fundraising Promise, which says there must not be 'undue pressure' to make a donation," said one fundraising manager, who asked not to be named. "That phrase is very open to interpretation."
Bonnie Green, FRSB spokeswoman, said it would consult on improvements.
"I think when some members actually got the form to fill in, they found it hard to pin down what was a complaint as opposed to a query or a concern," she said.
"Many charities had no problems, but we want to get it right for all.
"New schemes need to bed in, and this will be part of that process."