In "The Postmaster," the postmaster would call Ratan who waited outside the house ;but she never came at once. Explain why.

This is a small detail that Tagore uses in order to bring
out the character of Ratan.  It is really minor, but like so much in Tagore's writing,
the minor and small elements occupy great importance as they reveal much about the
character.  Ratan hears the postmaster's call.  She deliberately does not answer.  One
reason she does not immediately answer is because she is so very happy to be needed. 
Ratan is an orphan in the village, and has not been in a situation where she is needed
or even included.  In not immediately responding to the postmaster, she allows herself
the need to feel wanted, something that has been lacking in her own life for quite some
time as she has been an orphan.  At the same time, there is an existential reason why
she wishes to hear her Dadababu's voice again.  Ratan has been used to silence all her
life.  The lack of any human connection has created a setting where silence is her only
companion.  In hearing someone call her repeatedly, Ratan experiences noise and sound
where there had been silence or emptiness.  This is the reason why she yearns to hear
his voice more than simply one time, for it removes the vacuum of emptiness that her
life had been before meeting the postmaster.