This is different from a one-time shift after a method change. Wandering retention during a sequence usually means the column is not equilibrated, the column oven is not stable, premixed solvent is evaporating, or the proportioning valve is not delivering a repeatable composition.
Keep the solvent bottles closed. Give gradient methods the same equilibration time on every injection. Confirm the oven is on and at the set point.
Call Mahut if retention wanders while temperature and a freshly prepared mobile phase are stable, especially if pump pressure wanders with it.