The President has been openly fueding with his own senior adviser for months, but the relationship worsened recently when McMaster said in a public forum with cameras rolling that it was "incontrovertible" that the Kremlin interfered in the 2016 American election and is already trying to do so this year, a fact Trump tries to pretend doesn't exist.
A National Security Advisor is arguably the second most important senior West Wing staff member, second only to the Chief of Staff. Considering Trump had utterly zero qualifications in the defense, foreign relations, and terrorism arenas, his National Security Advisor is all the more important.
Based on a number of published reports drawn from numerous sources inside the West Wing, Trump has largely all but ignored McMaster and only meets with him infrequently for short sessions. The President wants complex matters dilluted down to a few bullet points, a process that is virtually impossible and leaves him with a near complete lack of understanding of security and intelligence essentials.
McMaster was widely consider a voice of reason and sanity when he was first appointed. But now that he has been administratively neutered and is all but out the door, one has to wonder who Trump is listening to for intelligence and foreign policy advice.
If he's listening to anyone at all.