More explanations

Even if you consider yourself a cool teacher who explains everything in a way that even the stupidest person would understand you, don't be lazy to explain it to your students several times, and what is even more important try to do that in different manners. Sometimes, they really don't get it, but it's not because they are stupid: maybe some of them just think different.
Be patient, and explain your material over and over again, making sure all students understand what you're talking about. You know, how difficult it will be for them to learn further, if they don't get the basis.