At long last, I solved the problem. The Autoretraction was enabled. I do not know how it is happened. I did research in the forum to have an idea to solve the problem. Finally, I disabled Autoretraction by sending the code "M209 S0(Disable Autoretraction)", so the manual control of extruder motor is working now, but I have not seen that retraction is working during the printing. Hopefully, it will work. Thanks people who contribute this forum. Thank you Repetier for creating this forum and firmwares also.