Long list of reasons, anything from the target field in the table being defined as NOT NULL to row redirection in your components. Use the dataviewer on the dataflow pathing and see exactly when those rows fall out, that will help you narrow down to which exact component is causing the issue. Then check the error conditions for that component and see what it does on failures.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
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