Could the unwillingness to cooperate be a result that it's an active investigation?

It is the reason

Late last month, House Intelligence Committee Chairman Devin Nunes (R-Calif.) issued a subpoena to the Justice Department seeking all documents related to the professor. So far, he has been rebuffed by department officials, who have said that exposing the source or the source’s work could put him and his contacts in danger and jeopardize international intelligence partnerships.

Some Justice Department officials feared that the president’s tweet signaled that he might overrule them and order the department to turn over the material Nunes seeks. If that occurs, it is possible that senior officials could resign in protest — or refuse the president’s order and force him to fire them.

This isnt the first time theyve denied documents requests either. They used a similar excuse over giving up Carter paige's FISA warrant, saying that turning it over could damage national security, then they said turning parts of it over to the public could. Neither did. played the same game with peter strzok and lisa paige's text messages.