In the ongoing attempts by Democrats to impeach Donald Trump, the media reports that the White House has refused to cooperate with the probe. Example. As I understand things now there are witnesses the Democrats want to talk to, but who are refusing to testify. The Democrats have issued subpoenas (which, again as I understand it, force the recipient to testify), but several witnesses have refused to testify anyway.
What happens if a subpoena is defied? Presumably there must be some consequence for this, and the linked source says that Ex-Trump aide Kupperman refusing to testify may "warrant a contempt proceeding against him". Wikipedia's article on Contempt of Congress says that in this case, the offender is "arrested by the Sergeant-at-Arms for the House or Senate, brought to the floor of the chamber, held to answer charges by the presiding officer, and then subjected to punishment as the chamber may dictate", with the typical punishment being imprisonment. However, if any of the subpoena'd witnesses have actually been arrested, let alone been imprisoned, I've not read about it.
Question: If the above picture is correct, why hasn't the media reported on the arrest of any of the witnesses? If it is incorrect, what actually happens?