I allow my students to retake a test they score poorly on provided they prove to me they worked hard to understand the concept better. Students are required to fill out this form explaining to me why they think they scored poorly. Then, they attach two proofs of something they did, on their own time, to better understand the concept. For example, a student may attach copies of rewritten notes, notecards, diagrams or graphic organizers they made, my signature of their attendance to a tutoring session, a YouTube video tutorial page, or others.
The form can be customized to fit your needs. This form is a great record-keeping tool that documents remediation.