How can it be that the tests are getting lighter?

How do you know if you've had your ovulation spike?

Temperature check? Have you used ovulation tests before? Or are you seeing a gynecologist?

Apps are not reliable. Not at all. It's just a prediction based on a cycle; if your luteal phase is shorter or longer, it won't match up anyway. Do you have a regular cycle? If so, you can "indeed" (in brackets) assume ovulation on day 14. But I have a cycle that's as variable as the weather—one time it's 26 days, then 33. I can't rely on it.

So your ovulation spike could have already happened, but it doesn't have to be. It's also possible you don't get the "typical" strong positive, but your test still gets dark; some girls never get a test line that's as dark or darker than the control line, even though they did ovulate. It depends on how much LH you have in your body. If I may advise, I'd use a more sensitive ovulation test.

Also, I'd keep testing for a bit... the Kruidvat tests aren't the best; if you read around sometimes, you'll see they can stay positive even when there's no ovulation or pregnancy, so ovulation tests can also stay dark.

Good luck, and hopefully you'll get your answer soon.