Two Possibilities:
1. Did you first bank the March 2020 points into the March 2021 use year (which you can do online)? Those points could not be used in any June 2021 reservation unless they were already banked. (if that is the issue, bank the points and then make the reservation, or, if reservation already made, do a modification to the same reservation you already have and you will be able to use the banked points as part of the modification.
2. The online reservation system, when there are banked points , sometimes initially shows when you first click to do the reservation that banked points have not been used first. Solution is to just continue and complete reservation and at the end you will see that banked points were actually used first.