The same thing as with Supreme Court. Presidents may appoint justices that are viewed as conservative/liberal but when they start ruling on their own and join the opposite ideological bloc, there isn't anything President can do.
FDR, when faced with conservative Supreme Court blocking New Deal, threatened "court packing" scheme - simply appointing pro-New Deal justices to achieve majority. A very drastic measure, but within the limits of US Constitution. The threat itself worked and Supreme Court began ruling as FDR wanted.
But Trump doesn't have that option with Powell. He can only sit and wait for Powell to go away.
An excellent analogy. Court Justices often rule against the "party" who nominated. Justice Roberts, a Bush nominee, has gone against republicans on a number of major issues.
Many are asserting "just cause" in the Federal Reserve Act. Disagreement in policy is not just cause.