From the Mueller report: (volume 1, page 10):
The investigation did not always yield admissible information or testimony, or a complete picture of the activities undertaken by subjects of the investigation. Some individuals invoked their Fifth Amendment right against compelled self-incrimination and were not, in the Office's judgment, appropriate candidates for grants of immunity. The Office limited its pursuit of other witnesses and information-such as information known to attorneys or individuals claiming to be members of the media-in light of internal Department of Justice policies. See, e.g. , Justice Manual§§ 9-13.400, 13.410. Some of the information obtained via court process, moreover, was presumptively covered by legal privilege and was screened from investigators by a filter ( or "taint") team. Even when individuals testified or agreed to be interviewed, they sometimes provided information that was false or incomplete, leading to some of the false-statements charges described above. And the Office faced practical limits on its ability to access relevant evidence as well-numerous witnesses and subjects lived abroad, and documents were held outside the United States.
Further, the Office learned that some of the individuals we interviewed or whose conduct we investigated-including some associated with the Trump Campaign---deleted relevant communications or communicated during the relevant period using applications that feature encryption or that do not provide for long-term retention of data or communications records. In such cases, the Office was not able to corroborate witness statements through comparison to contemporaneous communications or fully question witnesses about statements that appeared inconsistent with other known facts.
Accordingly, while this report embodies factual and legal determinations that the Office believes to be accurate and complete to the greatest extent possible, given these identified gaps, the Office cannot rule out the possibility that the unavailable information would shed additional light on (or cast in a new light) the events described in the report.