There are four usual suspects, mate, and they're easy to tell apart once you know what you're looking at.
It's a warm white, not a pure one. Most heritage whites are deliberately warmed with a touch of yellow, red or grey so they don't feel clinical. F&B's Slipper Satin and Pointing both carry a soft yellow undertone — gorgeous in a north-facing room with cool light, but in warm afternoon sun or under old-school halogen bulbs they can read distinctly creamy. That's not a fault, it's the colour. If you want crisp and cool, go for Farrow & Ball Wevet or Strong White, or Little Greene Loft White, which all sit much cleaner.
Your lighting is doing it. Warm white LEDs (2700K) and old tungsten bulbs push everything yellow. Swap to a 4000K neutral bulb and that "yellow" white often snaps back to looking white. Test your paint under the light you'll actually live with — daylight, lamps, the lot — before you commit.
It's oil-based paint yellowing. This is the big one on trim. Traditional oil-based eggshells and gloss yellow over time, especially in low-light spots — behind doors, inside cupboards, anywhere UV can't get at them. Nothing you can do to reverse it. The fix is to switch to a modern water-based trim paint, which holds its colour far better. Dulux Trade and Crown both do solid water-based eggshells for woodwork.
Or something's bleeding through. Nicotine, water stains, grease and old wood knots all leach a yellow-brown tint up through fresh paint, no matter how many coats you pile on. The only cure is a proper stain-blocking primer first — Zinsser BIN is the gold standard, shellac-based and blocks pretty much anything. One coat of that, then your topcoat goes on clean.
Work out which of the four it is before you do anything. Check the wall in good natural daylight first: if the yellow disappears, it's your bulbs or the white's own warmth doing the talking. If it stays put — especially in patches or on old woodwork — you're looking at ageing oil paint or a stain coming through, and that's a priming job, not a repaint.