If it’s not the wind, the flickering is usually due to an imbalance between combustion and fuel supply at the flame. The trigger is often a wick that is too long.
A candle works like this. The flame heats the wax in the wick, which then vaporizes in the flame and above all burns. The yellow in the flame is nothing more than the burning wax particles. Liquid wax flowing from below replaces the wax that burns. In the perfect case, with a quiet candle, there is a balance. The flame continuously burns just as much wax as flows in. Flickering happens when this balance is disturbed.
The wax deficit can trigger flickering
Suppose the wick is too long; a situation can then arise in which the flame has a fairly large amount of fuel available, specifically all the wax that is in the long wick. If it has a lot of fuel, it will flare up more. But because a lot of wax is burned in a short time, there is a deficit in the long wick, because the liquid wax cannot flow through the long wick so quickly from below. Suddenly the flame is undersupplied, collapses for a short time, and becomes visibly weaker.
In the meantime, however, liquid wax continues to flow into the wick, and so the deficit of wax in the wick soon becomes a surplus again. The flame flares up again, and so it goes back and forth. It’s like the stalling of a car engine that’s not properly timed.
Shortening the wick only helps in the short term
You can remedy this by shortening the wick a bit. But that often only helps in the short term. With the candles where the wick gets too long, this usually happens again and again because the thickness of the wick is not optimally chosen.
Candle in the freezer
In certain circumstances, it can help to put the infused candles (https://worldinfusedcandles.com.au/) in the freezer before lighting them. Because when the candle is cold, the wax needs more energy to melt. In this way, not so much wax flows into the wick. The candle burns slower. In the best case, this brings the ratio of combustion and the wax that flows back into balance so that the candle no longer flickers. But that’s not a guarantee.