Gravity TT Encoder Kit gives always same value

Hi, I am using exactly the same code as https://wiki.dfrobot.com/Wheel_Encoders_for_DFRobot_3PA_and_4WD_Rovers__SKU_SEN0038_ 

Only difference is interrupt pin number. I am using Arduino Mega 2560, I wired encoders to Digital Pin 19,20 which are interrupt 3 and 4. However, either motors are turning or not turning, I am always getting the same number: 532 and 1000. Does anyone knows why it is happening ?