Rolling problem and questions

I'm almost embarrassed to be mentioning this, seeing as this is a "professional" forum, because the problem I've been having with some of my walls is not very professional-looking in my mind. Oh well, here goes...

When I look down a wall about a day or two after I've painted it, doing some QC, I'll sometimes notice a thin, vertical line every now and then with a different sheen/lustre than the surrounding paint. I suspect that perhaps when I back-roll an inch or so into the fresh paint, I might be doing so with a dry roller or something. Or maybe too wet?

When I look straight on, there's nothing strange at all; the coverage is perfect. It's only noticeable when light is reflecting off the wall at an angle. Maybe this is normal and I'm being too anal, but I think this is unacceptable (I'm a perfectionist!).

What are your thoughts about what causes this problem? And what tips can you give for back-rolling that will ensure a perfect wall? Thanks.