Ovulation test not showing positive

I'm just after some advice. I'm on a 28 day cycle currently at CD16. This is my first month testing ovulation. I've brought cheap tests from the pound shop and am testing once a day. I have had no positive reading. Has anyone else experienced this? It's really worrying me and I know stress is no good when ttc but we need to test for ovulation to have the best chance to conceive. It's a catch 22, any help and advice would really be appreciated. Thank you.