theres a lot of factors.amperage, voltage, concentration of electrolyte, temperature of electrolyte, proximity of anode to cathode, size of anode, size of cathode, density of the clay pot all can play a role in the reaction. is the pot just a standard type pot without a coating of any kind? did you soak the pot in the electrolyte well before starting the experiment? trapped air in the clay could slow the reaction. if i had to make a guess, it sounds like the anode and cathode is too far apart. im not the best person to give advise on this but thats what it sounds like to me.