Yep. Keep testing. That's the thing about OPKs ... there is really no way to tell when it will turn +. I mean, if there was, we wouldn't need them would we?
I'm guessing you want to know why it's not getting darker every day. It is kind of weird to see it stay snow white for days and days when it's yourself you're talking about. But that is normal. Not everyone has a line all the time and not everyone has a "buildup". You might get a light line tomorrow and then a full-on positive the very next day. We're all different.
If you really want to use OPKs, keep testing.
You never know. It could turn + tomorrow or a week from now.