It can get down to your testing procedure, its very uncomfortable sticking the bud up your nose, so a lot of people dont want to make them self uncomfortable, so they are either too quick or don't stick it up far enough
sometimes its best to get some one else to do it for you
also, some of the tests are inconsistent, they are mass produced and there can be faulty ones, some are faded and need to be done again and sometimes the second line, the one that means you are positive, can be very faded if your body is fighting the virus off so your level of infection is weaker as its relative to the boldness, or darkness, of the Covid line