Six ways people waste a perfectly good serum
A lot of serums that get written off as useless were used in a way that guaranteed the result. None of these are difficult, and most people are doing at least two of them.
Putting it on bone dry skin
A humectant draws water from whatever is nearest. On damp skin that is the water on your face, which is the plan. On dry skin in a dry room it can pull from deeper down, which is the opposite. Pat your face rather than towelling it and apply while it is still slightly damp.
Not putting anything over it
Holding water at the surface only helps if the water stays. Something slightly richer on top slows it leaving overnight. A serum on its own in a dry bedroom has lost most of its work by morning, and this is the most common reason people conclude hyaluronic acid does nothing.
Applying it through your palm
Serum in a hand gets absorbed by the hand. A dropper into the palm and a rub across the face loses a real proportion of every application, which is also why people stop at the jawline: there is not enough left to keep going.
Using it when you remember
Three nights a week is not a routine, it is a series of unrelated evenings. The hydration effect holds while you keep it up and fades when you do not, so an every night product used sometimes is being tested under conditions where it cannot show anything.
Layering it under an exfoliant
A surface that has just been scrubbed or acid treated holds water poorly for days afterwards. Adding a hydrating serum on top does not cancel that out. If both are in the routine, the answer is usually less of the second one.
Giving up at two weeks
The hydration part shows within days, so if there is genuinely nothing after a fortnight of nightly use, that is a fair answer. What people usually do instead is switch at day ten to something else and never get a clean read on either.