Suds do not cut through grease and grime. It is more of a signal of the ability to clean than actual cleaning action itself. As Neal indicated, a cleaning formula usually has to have some kind of surfactant (surface active agent) that helps the cleaning solution make good contact with the surface and to penetrate/emulsify any greasy soil. Water is the real cleaner here but greasy soils repel water; the surfactant helps the water stay in contact with the grease. You are adding washing soda which gives a high or alkaline pH to your cleaning solution. Alkalinity can react with fats in a chemical reaction called saponification that breaks down the fat and makes it easier to remove. (I hope you're using gloves depending on how much washing soda you are using - it could be quite caustic to your skin.) Surfactant both makes bubbles and interacts with the soil. If it's in bubbles, it's not with the grease and vice versa. You know all the surfactant is "occupied" with greasy soil when the foaming stops because there is no "leftover" surfactant (surfactant not associated with the soil) to make bubbles. Therefore the foaming serves mostly as a way of telling you that you need to add more cleaner rather than being the cleaning action itself.
The Castile soap you are using is also a surfactant (as is all soap) but soap is a weaker surfactant for cleaning, especially if you have hard water. The positive ions in hard water (Mg+2 and Ca+2) can precipitate (make insoluble) soap and thereby make the cleaner less effective. It would also depress the foaming. You can use a commercial formulated cleaner (laundry detergent, dishwashing liquid, shower gel, hand soap, etc) to get the more powerful surfactants that will stand up to hard water and also give you the increased foaming that you want. I caution you here that you must be careful when mixing any kind of cleaner with other ingredients to avoid unwanted and possibly dangerous reactions. In this case you are using washing soda or sodium carbonate which should give your formula a strong alkaline (greater than 7) pH. Laundry detergents and HAND dishwashing liquids are also usually formulated at alkaline pH so they would be compatible with your formula. (Do NOT use a product formulated for the automatic dishwasher.) These products are usually about 10-20% surfactant, but the surfactants they use are more powerful than soap (in terms of foaming and grease removal) and you should not need more than a teaspoon or so for 20-30 oz of solution (5-10 mL or less for 750 mL solution). You don't want the formula foaming so much that you spend all your time trying to clean up the foam after you are done cleaning the grime! If you find this does not give you the amount of foam you want you can increase the amount but be careful - you don't want to get a mountain of foam. You can add the detergent along with your Castile soap if you want - testing by some organizations has shown that soap can be especially effective on grit or particulate soils even if its foam is weak.
If you want to know more about surfactants you can look at the Wikipedia article although it does contain a bunch of chemical jargon: Surfactant - Wikipedia
Hope this is helpful.
Karen
