Hello there sleipnir, and welcome to sc2mapster.
Things hasn't been the same here, since the web migration and everyones PM was deleted.
But I will try to help as much as I can.
First, you need to know if the unit in question have the proper attach point.
For example, I will show you a ghost attach point.
Picture 1 :
1. Open a map [any map is fine].
2. Click on the unit tab [U].
3. At the new tab type Ghost, scroll until you find the unit.
4. At the unit tab, scroll until you find model, click on it.
5. Double click on the Ghost.m3 and a new tab preview will open.
6. Click on the "view in cutscene editor" and wait.
Picture 2 :
1. In the cutscene editor, click render, and enable the view attach point.
2. Press SHIFT + D to open the model data tab.
3. Click on the Ref_Weapon, and you can see the attach point of the model.
The game will launch the weapon projectile model [missile, bullet, beam, etc] at the unit weapon attach point, in this case, the Ref_Weapon.
Ok, now you know how to see a unit available attach point.
Next, try to open your unit, see if that unit have a weapon attach point.
If not, the data will launch the weapon at default available attach point of the unit, which is the ground/origin.