I only found reason for the error and solution to it. http://scn.sap.com/message/14609181#14609181
The rpt which does not displays page number properly(i.e displayed 1 of 1+ in viewer) had page number field used in it. But there was no field which triggers total page count. So i was getting 1 of 1+.When i reach last page it gets the page count so when i click previous page it updates page number properly in viewer.
So it can be solved by using TotalPageCount some where in the rpt file so that it updates it at the beginning it self when displaying first page.