FRER not getting darker but wondfo are?

So I’m just curious if anyone else has went through this. I know line progression can be nothing and all the tests are different as well as urine samples I get that. I’m still wondering... This is my third pregnancy and my lines have always been super dark by now (15dpo) on FRER tests, so I’m just curious what others have experienced. I tested on a FRER at 12 DPO and today 15 DPO and the lines look the same. Not as dark as the control line like I expected.

I’ve attached a photo of my cheap amazon tests (wondfo) which clearly are getting darker. I’m just wondering what’s up with the FRER!? Has anyone else experienced this? Did I just get a bad box?

Thanks 💕