Why can't I delete a data module in Arbortext CSDB?

I am using Arbortext CSDB for S1000D (4.2 M030) and do claim to be an expert.  I have been deleting DMs, importing and exporting them quite happily until now.  Two data modules refuse to delete.  When I right click and click Management thed Allocate Author and Reviewer are greyed out, which they aren't on all the other files.  When I try and do Delete DMC I get: "Not authorised to Delete DMC" even though I've deleted all the others around them.  Any ideas?  Thanks.