This is my first time testing strips with that are cheap. I started earliest that I was possibly fertile and I followed the directions of submerging for 3-7 seconds and checking at 3 minutes but waiting until 10 minutes. I never saw a line except one day but it was so light it was barely visible. Do I need to buy the expensive ones or am I doing something wrong? I even tried not to drink 4 hours prior to the test to make sure it wasn't diluted. It was kind of defeated. Advice?