Test stop working after couple days!

I have been using the Advanced Digital Ovulation Tests for the first time. I have used couple of them without problems but last two days the tester did not turn off even after I put a new stick in to test the next day. These tests are very expensive and I would really like some help understanding what I am supposed to do. You guys have to understand this is something important for so many people who are trying to get pregnant depending on these test. This is unacceptable.