Sun spots, PIH, and evening out tone.
Every pigment-fading active runs against a re-pigmenting stimulus, and for melasma part of that stimulus is visible light — which broad-spectrum sunscreen does essentially nothing about unless you can see it on your face.
'Dark spots' bundles at least five distinct processes that sit at different depths, respond to different things, and have wildly different prognoses — one clears on its own, one relapses forever, and one will not move for any serum ever made.
Leftover acne marks come in two flavors that need almost opposite treatments — and mixing them up is why people spin their wheels for months.
Every brown mark gets lumped together as a 'dark spot,' but the four common types have different causes and answer to different treatments. Misidentifying yours is the most likely reason your serum 'isn't working.'
For melanin-rich skin, the breakout is the short part — it's the brown shadow it leaves behind that runs the show.
Darkening in the underarms, groin, and inner thighs is extremely common, almost never a hygiene failure, and occasionally a health clue worth reading. The shame around it is the biggest obstacle to treating it.