Hi
Im not sure how it varies between brands but mine predict an increase in your hormone levels that suggest ovulation in next 24-48hours. Maybe your hormone levels take abit to fall. Im not sure how long its ment to take for them to drop to a point where they are undectectable by OPK I norm dont keep testing once I get a +ve, just bd every day, usually for 3-4days depending if we have any time off together HTH