Surprise, you're not well informed... again...
2 minutes of google tells me that, at a minimum, the following have been subpoenaed but refused to testify
on orders from the White House.
Mick Mulvaney - Acting White House Chief of Staff[footnote]https://www.nytimes.com/2019/11/21/us/politics/impeachment-witnesses.html[/footnote]
Charles Kupperman - Former Deputy NSC Advisor[footnote]https://abcnews.go.com/Politics/obey-white-house-subpoena-witness-impeachment-probe-asks/story?id=66636082[/footnote]
Donald McGahn - former White House Counsel (separate investigation than the main impeachment proceedings)[footnote]https://abcnews.go.com/Politics/obey-white-house-subpoena-witness-impeachment-probe-asks/story?id=66636082[/footnote]
Also, honorable mention to John Bolton, former NSC Chief, who has been asked to testify (though no subpoena yet) and refused.[footnote]https://www.latimes.com/politics/story/2019-11-22/bolton-dangling-testimony[/footnote]
Finally, I have to ask whether you understand that a formal subpoena is not required to testify. This is rather basic, but a subpoena is a method of
compelling testimony. There are invitations and requests for testimony sent out prior to resorting to a subpoena.
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Late find (still under 5 minutes of searching here) of more people who refused to answer subpoenas:
https://www.cnbc.com/2019/11/04/four-trump-officials-refuse-to-testify-monday-in-impeachment-inquiry.html said:
The four who defied the subpoenas are John Eisenberg, legal adviser to the National Security Council, his deputy, Michael Ellis, as well as Robert Blair, a top aide to acting White House chief of staff Mick Mulvaney, and Brian McCormack, an aide at the White House Office of Management and Budget who previously worked for Energy Secretary Rick Perry.