I have an MS project Macro which unlink and links predecessors and I get the error 1004 on some project plans but not others, I have compared everything I can think between the plans and all seems similar with no obvious difference.

Has anybody had this issue or have any ideas as to what could cause it.

Things I have checked are
1) The predecessor exists
2) it is valid
3) the task has no other constraints
4) there is no resource conflicts
5) the links are one to one, so there are no multiple links

Any ideas/pointers would be appreciated.