For certain omissions made through forgetfulness, sujūd al-sahw is legislated. The underlying prayer has been performed, but a specified deficiency is remedied by an additional prescribed act. This is compensation for a shortcoming rather than a new performance of the entire prayer.
Leaving an essential pillar such as bowing or prostration cannot automatically be repaired merely by adding the prostrations for forgetfulness. Essential pillars and compensable duties remain distinct.
Diagnostic question: Is the original act absent, or has it occurred with a shortcoming in a required component?