I'm trying to add a dissolve transition between two clips, but when I drag the effect to the timeline I cannot place it where it is supposed to go. A 'X' is displayed, meaning I cannot add a transition at that particular point. I know the problem is not the length of the clips. One is 7 sec. and the other 3 sec. I remember I had this problem on my machine at home and someone told me I had to adjust a setting. I would appreciate any help.