DataDeploy sets column values to NULL

System

We have a problem that DataDeploy is erasing other data from our database when we deploy a DCR.
We have a table where several of the columns are deployed to and several are not (they are fed from another system). when we do a DD the non DD columns are all set to NULL. This only happens when the 'deployed to' table is not a 'root-group' e.g. a replicant.
Anyone have a solution?

cheers

/Nick