It is because you have 3 GUI elements all connect to (x,y) and there are all dragable.
1. Particle
2. mg
3. rod
So only the last one will receive mouse drag action.

The above order is the same order shown in the EJS GUI tree.
So the action for [i]mouse move to (x,y) and drag[/i] was assigned to [u]rod[/u].

Because your action code is assigned to [u]Particle[/u]
What you can do is
1. Change the above order so the [u]Particle[/u] appear as the last GUI element
2. Change the dragable property for [u]mg[/u] and [u]rod[/u] to [b]false[/b]

Remember: If there are several GUI elements assigned to the same variable, then the last one's action will be the one being executed.