Not Getting Darker?

Gabi

I'm a little confused. Now 5 weeks and 2 days pregnant, have been testing from the beginning. Last test I took before new box of strips was 18 DPO, which was much clearer and darker than the previous tests. I got my new strips in yesterday at 21 DPO and went ahead and tested in the afternoon. Then again this morning. They all look the same to me. Is this normal? Am I being paranoid?