A plain candle has nowhere to hide. There is no fragrance covering the smell of the wax and no dye disguising the burn, so the only things left are the quality of the wax, the size of the wick, and whether whoever made it got the ratio right.
"Unscented" and "fragrance-free" are not the same claim
Unscented means no fragrance oil was added. Fragrance-free is a cosmetics term and, confusingly, is sometimes used for products that do contain a masking scent to cover a raw-material smell. For candles the distinction rarely matters, but it is worth knowing that an unscented candle is not odourless — paraffin has a faint petroleum note when it burns, and soy has a slight sweetness. If someone tells you a wax burns with no smell at all, they are selling.
Why ritual work usually wants a plain candle
Scent is a deliberate ingredient. If you are burning incense, or dressing a candle with oil, or working with a particular herb, a fragranced candle adds a second smell that fights the first one. That is the practical reason plain candles are standard in most traditions rather than any rule about purity — a scented candle is simply a candle with someone else's decision already baked into it.
The same logic applies at a dinner table. A scented candle next to food is competing with the food, which is why restaurants use unscented ones almost without exception.
Tealights, and the four-hour lie
A standard tealight is sold as a four-hour candle and a larger one as eight. Both are approximations made in still air at room temperature. In a draught, or in a holder that traps heat, a four-hour tealight can be gone in under three. In a deep glass holder that starves it of oxygen it may gutter out early with wax left in the cup.
If a tealight is burning fast, the usual cause is airflow rather than a fault. Move it away from the door.
Why candles tunnel, and what to do
A candle remembers its first burn. Wax has a memory: whatever diameter the pool reaches on the first lighting is roughly the diameter it will reach every time after. Put out a wide candle after twenty minutes and it will burn a narrow shaft down the middle for the rest of its life, leaving a wall of unused wax.
The fix is only available once. On the first burn, leave it until the melt pool reaches the edge — for most container candles that is one to two hours. After that the shape is set.
Wick trimming, which everyone skips
Trim to about 5mm before each lighting. A long wick produces a tall flame, a tall flame produces soot, and soot is what blackens the glass and makes people think the candle is faulty. It is not; it is untrimmed. Nail scissors are fine.