To make head or tail of something means to understand it, but in this context it says he has been running over this incident in his mind and couldn't make head or tail of it which means he couldn't understand it which is the opposite.
Context: He'd been thinking about the incident continuously and he still could very easily understand and make sense of it.
Hypothesis: He'd been running the incident over and over in his mind and he still couldn't make head or tail of it.”

Is the hypothesis incorrect? (Yes, no, or unknown)?