Forum:KB:how does obi-wan not recognize R2 on tatooine 0 BBY, he was anakin's droid

how does obi wan not recognize R2 when they meet on tatooine 0BBY he was anakin's droid for so long, how could he forget him, and on that note hows does lars not recognize c3po when he buys him

also how does vader not recognize his droids (escpecially 3po, when han gets frozen)