What is it, exactly, that is going to get underlings to flip? This is not a prosecutorial investigation, and Issa's committee has neither a carrot nor a stick with which to encourage comprehensive and truthful testimony from anyone not otherwise inclined to provide it.
Not everyone is willing to perjure themselves, especially if they think they'll get caught. If the Republicans win the Whitehouse next year, there could potentially be criminal indictments if they did. While I don't think that is likely for the cabinet level players, it could happen at fairly senior levels given the right set of facts and circumstances. It's this potential that might spur some to come clean if called to testify.