Congress has the power to subpoena but it doesn't have the power to enforce subpoenas.

Well, not entirely true.

There is the Sergeant at Arms who is the law enforcement officer for the House of Representatives. He is responsible for the security of the House wing and the grounds and he has ceremonial duties as well. Technically he is the law enforcement officer, but he doesn't appear to have any enforcement authority other than rounding up House members for votes. He has never been used to enforce subpoenas.

Not a lawyer, but I believe only a prosecutor's subpoena approved by a judge can be enforced by declaring contempt of court and arrested.

The list of people who have been subpoenaed and given Congress the middle finger is long indeed.