advise needed!!

Ashley
Hello!! My husband and I have been ttc for the past several months. I have been using clear blue advanced for the ovulation test. I bought a new box and for the past few days I have been getting negative readings. Well I came across the box from last month and I realized I had a test in there so I used the correct test strip with the correct and got a peak reading!! I double checked it with the new box and got negative again. Why am I getting such different readings??