The B757 has had issues with leading edge slats locking out over the years,
normally fixed by replacing L/E slat actuators.
Trailing edge flap locking up can be a very hard problem to sort, you can change a number components and all works on the ground.
Then a flight or two later bingo same problem, have known engineers pulling hair out and many calls to Boeing for idea's.
So you can see it is not straight forward.