WBS Phase

We have always not been able t go back and change time with minus hours once COSWIP has been processed as Epicor thinks you are then entering negative hours for a day. We have one project with 2 WBS phases attached to it and strangely one of them will let you post a minus retrospectively but I cannot see any reason why that is the case or any difference between the two WBS phases. Anyone know how this is enabled? TIA