Your students are updating the variable that they use to store the nuke's x property, but they aren't updating the property block itself. They will need to use the
setProperty block to affect the position of the nuke.
You might want to make sure the students are clear that this line of code
var nuke = getProperty("nuke", "x");
just stores a number (the current x property of the "nuke" element) into the variable
nuke. It doesn't create any lasting relationship between the element's x position and the variable.