When I was TTCAL, I had between 8-11 days of positives every month, no matter what brand I used. My doctor had never seen it before, but she also said most women stop testing after the first positive. So I started tracking BBT as well and found that I ovulated in the MIDDLE of my positives (aka 10 days of positives would mean I would ovulate around day 5 of positives). It was bizarre, but consistent. So if switching brands doesn't help, try charting BBT too. That's what finally helped me pin down my ovulation date.