Clearblue Advanced Digital Ovulation Kit Help!

I have been taking the Clearblue Advanced Digital opk's since the 30th of June. I have been getting high fertility since then. I have been using the same holder from my first box (I'm on my second box now) just like the instructions say in case 10 tests do not show peak fertility. This morning I decided to use the holder that came in the second box and it came up low fertility. So I took another test with the exact same urine, but with the holder that came in my first box and it came up high fertility. Do you think that the holder that came with my first box is faulty or just stuck on high fertility? I find it hard to believe that I've been in high fertility for 12 days.