Why is the 'Back' button disabled?

In PerfectIt 5, you can go back and review the previous error within one of PerfectIt's checks by clicking the 'Back' button. However, the 'Back' button only moves within a check, not to a previous check. For example, it will move back within the check of 'Phrases in Capitals', but will not go back to the check for 'Abbreviations Without Definitions'. As a result, when PerfectIt is at the start of a check, the 'Back' button is disabled or greyed out.