Flicker at low dim, steady at full. Usually a dimmer designed for halogen loads driving LED fittings it was never matched to. The fix is a trailing-edge LED dimmer sized to the fittings on the circuit, not another fitting.
Buzz or whine. Usually a dimmer mismatch as well, or an old transformer under load from a retrofit lamp it was never built for. A fitting that hums on its own is on its way out.
Slow start, or a faint glow when switched off. A driver taking a moment to charge is normal for some fittings. A ghost glow after the switch is off often comes from an illuminated switch or a long shared neutral run and is usually not a fault, but we check it, because the same symptom can be a wiring fault.
Dead, with the others fine. The driver has failed. One new fitting, matched to the rest.
The whole circuit dead. Not a downlight problem at all. Breaker, switch or a connection in the roof, and worth a call before you buy anything.
Every visit starts with this sorting, because it decides whether you need one fitting, a dimmer, or an hour of fault-finding. The office quotes it once we know which, and if the answer is a dimmer rather than the six fittings you were expecting to buy, that is what you are told.