I had the same problem, the first time i mixed pyrocat.
First the bad news - at this point, you can stir until the cows come home and the potassium carbonate will never dissolve.
Now the good news - get the potassium carbonate into suspension (stirring rapidly) and pour 10 or 20 ml into your working volume solution (part A developer and water) and it will dissolve readily and work fine.
Going forward, the secret to getting the potassium carbonate to disolve the next time you mix the powder is to slowly (over the course of 5-10 minutes) add the powder into 100ml of moving water. it will dissolve and stay dissolved.
hope this helped.