Please, give a link to your encoder device. How many encoder devices are connected with the Slave?
Also, provide the connection diagram of the encoder device with DPin-2, 3 of the Slave.
You have enabled the internal pull-up at DPin-2 (INT0).; so, the default logic is always HIGH. Therefore, the trigger mode would be either FALLING or LOW; it can not be RISING.
One revolution of the encoder is equivalent to 6.1 inch. During one revolution, how many pulses the encoder generates.
To me it appears that something is not correct in your ISRs of INT0 and INT1.
If possible, provide the signals diagrams of the encoder when Motor turns CW and CCW.