"They made a re-jigged bid rather than a new bid so they have changed it around to make it look prettier," manager Iain Dowie told the Coventry Telegraph about Forest's latest offer.
"There is something on the table but it is not quite what we want.
"But Leon won't be moving anywhere unless I can get similar or better cover in."
Celtic meanwhile have agreed a fee with Dundee United for midfielder Barry Robson, subject of a seven-figure bid from the City Ground earlier this week.