Undertrading is the quiet failure. It produces no blow-ups and no stories, just an equity curve that never goes anywhere and a trader who concludes the method does not work.
The causes are usually fear rather than patience: a recent loss, capital that matters too much, or an unresolved drawdown. It is often the phase that follows a large loss, and it can persist for months. See hesitation and scared-money.
Measure it before fixing it. Log every setup that qualified, whether or not you took it, and compare taken to available. If the ratio is low, the problem is execution rather than strategy, and the answer is smaller size with full participation rather than bigger size occasionally.
Related: hesitation, scared-money, impostor-syndrome, process-goals