Gems, fatigue and increasing magic levels

[Reay]

What are the rules with using gems to increase magic levels and fatigue?

I know the basic rule is you can use up to N gems (where N is your level) to increase your level by 1 or reduce fatigue.

I scripted the spell Returning (Astral 2) as the first and only spell for an Astral 1 mage with 4 astral gems, and the AI would not cast it.

The spell costs 200 fatigue so I thought it would use 2 gems to cast it, plus 1 to reach Astral 2. Does the 2 gems use to cast the spell affect N or is there some other reason why it would not cast Returning?