What people actually mean by glow
Glow is the word doing the most work in skincare marketing, and it covers at least four different situations. They photograph alike. They behave nothing alike on a face, and they answer to completely different products, which is why so many people end up with a shelf of things that all promised the same result and delivered four separate disappointments.
Four things, one word
Light off a smooth surface. Your outer layer sheds constantly. When dead cells pile up on it, the surface turns rough and uneven and scatters light instead of bouncing it back cleanly. That reads as flat, or tired, even when nothing's wrong underneath.
Water. Skin low on water looks duller and less plump than the same skin properly hydrated. Lab work on sugar humectants measured the mechanism directly: water driven into the keratin of the outer cells plumps them and changes the way they scatter light.
Even tone. Melanin is the pigment that gives skin its colour. When it builds up unevenly you get dark spots and patches, and tone reads blotchy however smooth and hydrated the surface is.
Shimmer you apply. Mica splits into thin transparent flakes that catch light, usually coated with titanium dioxide or iron oxides for a pearl finish. On skin it sits inert. It changes the photograph and not the skin, which is fine as long as that's what you wanted.
Four different fixes
Surface roughness answers to gentle exfoliation, used in moderation, plus not stripping the skin in the first place. Over-scrubbing takes you backwards.
Water answers to a humectant such as glycerin or hyaluronic acid, followed by a moisturizer to hold it where you put it.
Tone answers to melanin-controlling actives, with daily sunscreen as the load-bearing part and not the optional extra. This one runs on a different clock from the other three. Depending on which active you pick, you're generally looking at somewhere between six and twelve weeks of consistent twice-daily use before there's anything to see.
Shimmer answers to cosmetics, and it works the moment you put it on.
Why the wrong bottle disappoints
A brightening serum works on pigment production. Point it at dullness caused by dehydration or by surface buildup and it's got nothing to act on, so it does very little, and you wait a couple of months to find that out. It runs the other way too. A rich hydrating routine won't shift a dark patch, because the pigment sitting in that patch isn't a water problem.
Most "this did nothing for me" reviews come down to that mismatch.
Which one are you missing
Stand somewhere with decent light and ask three things.
Is the flatness spread evenly across your whole face, or is it in defined patches darker than the skin around them? Even across the whole face points at surface or water. Defined patches, a mark tracing an old spot, or symmetrical shading across both cheeks, point at pigment.
Does your skin feel rough under a fingertip, particularly around the nose and chin? That's the surface answer. Does it feel tight, especially by mid-afternoon, and does it look better within an hour of a hydrating layer going on? That's the water answer.
Hold on to your result. The next two steps deal with dullness, which is the surface and water side, and they're the ones to concentrate on if your answer was even flatness. From there the course moves onto hyperpigmentation: what kind you have, why sunscreen comes before any active, how vitamin C fits, and how long fading actually takes. Plenty of people need both halves, and doing them in this order costs nothing extra.