Unfortunately, there is no direct way to fix such problems. The problem is caused because the missile itself has 0 Height value. And it's kinda impossible to increase that height.
Alternatively, you have two awesome solutions :
- Editing the model (Difficulty : Easy) :
1- All you have to do is go to Model Editor by Magos
(War3ModelEditor) and import the model you want as a missile.
2- In the top, click "Window" tab and choose "Model Editor". Now you should have some 4 square-windows of views. And Inside them , there are some red dots linked by white lines.
3- Select all the red dots (Vertices) [HOWTO : Left-click and drag], they should be green now. At the top, click "Transformations" tab and choose "Translate"...
4- Change the "Z" value to some great positive number (Like 50 or 100), play with it until you get a good height. (The Z value is simply the height of the model)
5- When you finish, save everything , and re-import the modified model into your map. It should have a new height now... If the height is still low, re-do the same operation but add more Z value...
2- A custom spell making (Difficulty : Medium) :
Making a linear-missile spell is the easiest spell level so far. So you should learn making custom spells, so you can control the missile (which is a dummy) as you like.
Good luck.