The scent throw you get from a candle depends on two things. Firstly, the volatility of the oil (i.e. how readily it evaporates) and secondly, the temperature of the base/carrier (wax in the case of a candle). It really is that simple. Everything else is much less important.
Paraffin wax candles typically have a hotter melt pool than soy candles, so less volatile oils will evaporate with the extra heat. In a soy candle, the wax is very thick (viscous) and heat does not transfer through the wax so easily. This is why large wicks are required in soy candles and the resulting melt pool is less hot. The cooler melt pool of the soy candle, results in less evaporation of the perfume, as there is less heat. It is this that results in the relatively poor scent throw of a soy candle (compared to paraffin and other less viscous waxes).
A simple experiment proves this. If you were to make a soy candle, with 8%-10% scent loading and got little or no hot throw, the same wax/oil mixture when used in a wax melter or oil burner would most probably fill the room with scent within minutes. Why? Because the wax is heated by an external source and gets hot enough to evaporate the perfume. Achieving the same amount of heat using a wick is very difficult if not impossible in some cases. This simple test proves that the oil is fine. It just isn't getting enough heat from the candle system. This is also why multi-wick candles tend to generate greater scent throw, as the melt pool tends to be hotter (multiple points of heat) and of course bigger.
So, to recap......you need a warm/hot melt pool (the temperature of which will vary depending on your wax and how well wicked it is) and an oil that is sufficiently volatile to evaporate in the molten wax. This is shown below...
|Low Volatility||High Volatility|
|Hot Melt Pool||MODERATE||STRONG|
|Cool Melt Pool||POOR||MODERATE|
Table 1 - Scent throw based on melt pool temperature and oil volatility
So....what can be done?
The secret to making soy wax candles succesfully is to find a wax that you can work with technically (surface, wicking etc) and then search high and low for oils that are suitably volatile for your chosen wax. Then you need to wick them correctly to get as hot a melt pool as possible whilst achieving a safe/clean burn. This can take time and a lot of testing, and all soy waxes (and peoples expectations) are different.
You must also accept the fact that not all oils work well in soy wax and be prepared to spend a lot of time and effort (and some money) testing new oils and wicks. Getting high qualiy (retail ready) soy candles is not something that should be underestimated. It takes time, resolve and effort; but the results can be wonderful. A different mindset is required than for paraffin candles where everything just seems to work.
This is an extremely complex area and there are other chemical factors that make using natural waxes problematic (compared to paraffin waxes), but having the correct mindset and a willingness to experiment and fail are essential.