I'm not sure how it looks for you, but here's what I do when I want to get a fireball:
* make sure I have all the "Attack from a distance" talents (Student, Apprentice, etc.) bought up to my general sign level - I'm not certain it's required, but I know it works when I have them equal,
* choose the Igni sign - on the endurance bar at the top left I now see two notches, one for the basic version and one for the distance version (the distance version costs more)
* point at the monster I want to attack (so the cursor changes)
* right click and hold until the diamond fills up,
* release - if everything went well, I get a fireball, otherwise (e.g. if there are obstacles in the way) I only get a red error message and no effect - but still lose Endurance.

First thing to check would be whether you have the two notches on the endurance bar - if not, something is wrong (the distance version costs more Endurance, maybe you only have enough for the basic version).

Second thing to check is whether you're pointing at a monster (you should see the attack cursor).

Third, do not release the button too early - you can hold the fireball as long as you want, so it's better to hold it too long than too short. I'm not sure if the diamond does something to signal when it's ready (even though I just used it yesterday).