GL Posting Issue

We have been running payroll in production since January'2011 in parallel run without GL posting. Now we are checking august payroll GL posting in simulation mode so that we can do actual payroll posting in month of september.

Now posting in august is causing problem. If there is a retro amount then all amounts of previous period are trying to get posted which is causing error. Even if the retro amount is 10 then the basic salary of the previous period is coming as line item in GL posting and causing error.