Flash cards -- highlighting -- writing down definitions. Do you do all that and still have low test scores? Often, straight memorization is not enough to really learn the subject well.
For example, in biology you may learn that proteins are formed from peptide chains. You may also learn that a polymer is a chemical compound that has repeated units. But if an exam question refers to a "polypeptide," you might not realize that it was talking about a protein.
The key here is to make associations. What are polymers? What type of biological compounds can be classed as polymers? "Poly" means "many." So, a polypeptide would be "many peptides." What compound is composed of many peptides? A protein, of course.
This type of reasoning is not developed by straight memorization. You need to reach for the meaning. Make lists, tables, or diagrams; look up words; make links and associations. Write definitions in your own words only. Don't guess. Investigate, ask questions, read other sources.
If you can explain it clearly to your mom (dad, kid brother, friend who's an art major), then YOU understand it as well.