Accidentals covering notes

I had MuseScore already a long time on my computer, but didn't use it until
recently. I have to say, it works terrific. But ... I encountered now a
problem, for which I didn't find a solution: one beam carries two notes - an
F and an E-flat. It happens now, that the flat is written on top of the note
before the E. I have tried to move something, but that was not possible. I
tried to enlarge the entire notation, but that left the problem as it is. How can I move
things, so that the "flat" stands free and not on top of a note? (It happened on
the page of the "violin part", in the complete score there isn't a problem;
everything appears correctly.)