Your proportions are correct for a 100 % solution. I happen to use sodium carbonate myself even though the formula calls out potassium carbonate. Judging by what Sandy King told me, I will pass along that potassium carbonate dissolves into solution much more rapidly then the sodium carbonate that I use (sodium carbonate reaches saturation sooner then potassium carbonate). Sandy informed me that in light of that inherent tendency that I should use 500 ml of water to the 100 grams of sodium carbonate. And that I should just use five times the amount of solution B in the mixture of the use solution. Perhaps you might benefit from doubling the amount of water in your case and using twice as much "B" as called out. The amount of carbonate in the use solution is the important thing...how it got there is immaterial.
I suspect that your problem is related to the temperature that you are using. Perhaps increasing the temp of your water to 95 degrees would help in dissolving the potassium carbonate. Hope that this helps.