Ah. Lore time.
My sprites (the earthbound sprites, not to be confused with nightmares’ Aeternum babies) all have a direct spiritual connection to the Spirit that made them. It is through these connections that a sprite can Pray, which is basically another word for incantation here. They borrow magic from their Spirit, and in the case of the wood sprites, they borrow magic from the Earth Spirit, the Lady of Life.
Sprites’ souls are essentially little pieces of the Spirit’s soul. Sam, as a human, has an entirely different kind of soul and thus no connection to the Spirit. He could try to Pray, but he doesn’t have any way to channel magic from the Lady of Life.
Cerul, one of the first wood sprites Sam meets in Brothers Asunder, has a gift called the Voice of the Spirit, which means that his connection to Her is the strongest it can be. The gift enables him to Pray much more without tiring, it gives him an empathy to the feelings, physical and emotional, of the living things around him, and it enables him to see where the Spirit’s influence flows. He would have been able to see right away that Sam does not have a connection to a Spirit.