Retesting a candidate should be a company-wide policy. You should first check to see if your company has instituted such a policy.
If such a policy is not in place, we would recommend establishing a uniform policy. Policies can be time-based, action-based, or a combination. Time-based policies indicate that retesting is available after a certain period of time. For example, job knowledge tests may be broad and deep in content and would require many months of remediation. Alternatively, a month or two may suffice for a simpler test, such as basic arithmetic. Action-based policies indicate retesting is available after remedial actions, such as taking a course or further on-the-job training.
We recommend that clients use the policy which best fits their circumstances and aims.