Adding a note removes existing notes

Hi,
First time user...
When I try to insert a whole-note as shown in the picture, then all the other (four) notes above & to the right on the second bar disappears:

I just cannot add a whole-note under the 3/4 note, or if I first enter the whole-note, I cannot enter the 3/4 note (then the whole-note disappears or reduces to quarter notes to the rigtht. Why is this? Am I doing something wrong?

It seems MuseScore has it's own way with notes...

//RIL