There are 4 things that could cause this
1. The unit firing the rpg does not know the target exists, ie well out of los
2. The unit may indager himself by firing at the target, ie a rock in close proximety to the shooter
3. The target does not exist / is out of range / or cannot be destroyed
4. The target is behind a static object or friendly unit.
First check the names are correct eg.
shooter dotarget whateva
~2
shooter dofire whateva
(thats another thing, dont JUST use dofire)
then... if that dosent work, put the units on a open plan back to back at some distance away and use the command to make sure the unit is destroyable.
If all that dosent work, i could help you out if you send me the mission.