Sometimes it's hard to mix. Are you using distilled water - that may help. I use sodium carbonate, and it's even harder to mix with water. I mix it in 5 times the amount of water, then, when mixing the working solution, add five times as much. That gets the right amount of chemical into the solution, and the added water is not enough to worry about.
With potassium carbonate, you might want to try doubling the amount of water for the stock solution. Then, if you are using the working solution 1:1:100, you'd actually use 1:2:100.