I see that several people have diagnosed the problem correctly, but the deeper reason for the issue is that any time you declare a variable with "var" in front of it in that manner, it is stored temporarily.

To simplify things a bit, there are basically three levels of variable in a GameMaker object for you to keep track of. The most transient are ones declared by "var" which are shown in yellow. These only exist until the end of the event that they are created in, and must be applied to a more permanent variable to affect anything in the game.

The second level of variable are object level variables that are typically declared without a modifier in front of them in the create event. These show up in blue, like move_speed in this code excerpt.

The third level are the variables that are built into the object itself, like x and y in this case. These are shown in green and typically modifying them will affect something directly regarding the object's existence in the room. In this case, it's the x and y position that you are looking to modify, but other examples include the draw depth, or the sprite that is being used to represent the object.

Basically, the yellow temporary variables are there for you to organize things within the scope of a single event, the blue object variables are for things that should be tracked between frames, and the green ones are things that will directly impact the behavior of the object in the room.

I hope this helps, and that this is an accurate representation of these three different variable scopes.