Teachers: how do you support students who grasp concepts in class but struggle during exams?

I’ve observed that some students participate actively in lessons and complete homework successfully, yet their performance drops noticeably on tests and quizzes. They often tell me they understand the content, but once the exam starts, they feel anxious, overwhelmed, or suddenly can’t recall what they know.

For those of you who’ve encountered this, what approaches or strategies have helped students feel more confident and access their knowledge under test conditions?