Martin cannot remember who invented flush toilets because he was flirting with a classmate when his history professor described this momentous event. His forgetting appears to be due to

ineffective encoding

Explanation:

Ineffective encoding could be defined as inability to process or restore information except they remember how they where stored. This is when the way information is encoded affects the ability to remember them. If a student thinks about the way he stores a concept in his book than reading them, he would remember them better when the exam is given.

This is the scenario Martin goes through when he is been asked question by the History professor