I see it now, sorry for that 
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I stated my question in such a way that, anyone could understand it by just looking at the first case.
Actually i used the 5th motor for my testing, which was connected to pin 9 and pin 10. I didn't connect anything to pin 1 and pin 2. I connected this motor to pin 9 and pin 10 and sent the alphabet 'u'(using the serial monitor), motor rotated 90 degrees, then i did the same 4 times and motor rotated 360 degrees. now just for testing purposes I sent 'uuuu' and pressed enter. After that the problem came out. The serial monitor showed that the 'u case is called 4 times, but the actual rotation of the shaft was about 300 degrees only (it was in between 270 and 360 degree).
Now I have told you everything.