There may be a few reasons:
Nasal Desensitization – When you’ve had the same candle burning for some time, your nose gets accustomed to it, something also referred to as nasal desensitization. Rotate fragrances every 3 or 4 days. Light another candle with a different scent for a couple of days, then go back to your original candle. Removing yourself from the element and then introducing yourself back into the element will allow your nose to recognize the fragrance again.
Vents – Is your candle burning right underneath an intake vent for a heater or an air conditioner? If so, it may be pulling all fragrance from your candle. The bulk of fragrance gets drawn into the ventilation system and filtered out
Air filtering system – Don’t burn your candle next to an air purifier ionizer. While they do a great job at filtering the air, they pull the candle fragrance out of the air, the same way they pull smoke out of the air.