Message combo box is not rendered upon selection of responder

When editing a test step, the user is asked to select the initiator, the transaction and the responder.
Then, a required field "message type" is displayed as an inputText. If you fill out this field and then click on the add button, if the transaction is based on HL7v2, your value in the "message type" field is erase and a combo box is displayed instead.
When this combo box has to be rendered, it has to been shown immediately after the user selects the responder !!! Otherwise it is really time consuming