Why didn’t Graf Spee sink all three ships?
At about 0628 Langsdorff was being straddled by the light cruisers and saw himself threatened by a torpedo attack from them and so shifted his main armament to them, since he regarded Exeter as no longer a threat to him.
Later he did turn his main armament back to Exeter but obtained no hits. By this time his main fire control director was out of action and things were becoming confused, especially due to the frequent hits from the light cruisers.
This may have been the time Langsdorff realised that he was defeated and so did not try to regain the initiative when Ajax’s two after-turrets were put out of action at about 0735 and the advantage was very much his.
In a letter to his wife, Joan, Harwood made the point that right from the start he had seized the initiative and had attacked when Langsdorff expected him to run. Hence Langsdorff lost the initiative early on and although afterwards he may have identified several ways he could have done things better, at the time his reaction to the unexpected aggression from the three smaller ships prompted him to eventually flee the battlefield.