False Negative??

I'm five days late and have been taking a test every morning since the day after first day missed. My period is usually like clockwork, the latest it normally comes is the night/next morning it was due. All the tests I've taken have all come out of the same pack so I'm wondering if it could possibly be a bad pack of tests and I'm getting false negatives? I don't know if that's possible but I don't understand how/why I'm still getting negatives this late.