Why did critics find Roderick Chisholm's agent-causation theory incomplete?

Critics found Roderick Chisholm’s agent-causation theory incomplete primarily because it failed to provide a detailed explanation or mechanism for how agents cause actions.1

While Chisholm’s theory sought to preserve the intuition that the ultimate origin of an action—and thus moral responsibility—lies with the agent rather than prior events, it treated agent-causation as a primitive, unanalyzable notion that could not be reduced to more basic terms. Consequently, many philosophers argued that the theory was unsatisfactory because it merely posited the existence of freedom rather than explaining what it is or how it is possible. Critics maintained that the theory left a "blank space" where a substantive explanation of the causal process should have been.1