Advise try reading a copy of Tom Morris' "Philosophy for Dummies," a good book about Western philosophy. Then, depending on how your teacher structures his or her tests (essays using main ideas, and giving your opinions, and/or, t-f or matching facts), study and prepare accordingly.
Inasmuch as you incorporate key ideas into your everyday awareness, such would likely provide memory help.
And, talk with your teacher about your study effort and seeming lack of recall. Perhaps you experienced a single event of test anxiety, perhaps related to a subject you find challenging?