Having problem checking in parts inserted into an assembly already in the vault.

My work recently installed PDMworks Workgroup and we are having a problem checking in new parts that are inserted to an assembly that we have checked out of the vault. A coworker of mine has started an assembly and our boss is wanting for all of us to work on it so none of us will have actual ownership of the whole project, only parts of it so we have the assembly checked out as a Read-Only document. When we create new components and insert them into the assembly ,that we have checked out of the vault, and then go to check in the assembly with the new components inserted, none of the new components show up on the list to be checked in. Does anyone know why this is and how to fix it?