The NFL does not have the power to subpoena
The issue at hand, according to the report, is that the phone in question where they requested in order to review the texts, was destroyed, and it was destroyed on the day that Brady was set to meet for the first time regarding the case. When the investigators requested access to the text messages, he and his agent refused to allow that to happen. Only when he went back to start the appeal process of the suspension did he inform them that the phone was in fact destroyed. Also, despite his claims that he routinely destroys his phones, the one prior to the phone in question was not.
Also, when the NFL presented them the opportunity to bring in the two other people in question with deflating the footballs, they would not allow the NFL to talk to them.
Take what you will from that