Dragging Execution Occurencies down

I try to model something with a sequencediagram but somehow i cant drag the execution occurrence down after i set them via a self message. this wouldnt be a problem if the bar would be instantiated on the point i drag the object to but the program just fucks it up everytime.

My question is if someone can tell me how to solve this or should ignore it and install an other program who is actualy capable in doing such standard things.

Im so mad right now because i cant even drag simple things such as bars to the position i want because the program "ai" doesnt except it allthough it has the right syntax. All this stuff costs me so much time...