Maturity
Little Tolly was Leonel's greatest reliance. In fact, even in comparison to Little Blackstar, the little one was Leonel's greatest partner. If Leonel prepared things for Blackstar, he couldn't possibly forget about Tolliver.
Leonel hadn't really done anything special for Tolliver in his lifetime. Outside of feeding the little one as many ores as it could manage, there was really nothing else to do.
At the same time, though, Leonel had to be careful about this. This was because he had never forgotten his father's first lesson about the hazards of Metal Spirits and how overeating could cause them to go berserk. Due to this, Leonel was very careful with how he fed Little Tolly and the little one's progress was quite slow, although consistent, as a result.
As of now, Little Tolly should still be in the Fifth Dimension, and the little guy wasn't close to the Sixth. This could be considered to be another limitation to Leonel's Force Crafting.