Benefits Contributions not calculated for the first payperiod

Hi,

We have  payroll run semi-monthly basis  the payperiods are generally first day of the month to 15th and then 16th to 30 or 31st

example: Employee John smith is hired on 05th august 2013 enrolled in savings and pension plan on that date - when I run the payroll for the first period of august say 01/09/2013 to 15/09/2013  the payroll does not calculate the EE and ER contributions for this pay period, however from next payperiod onwards the contributions kick in. How can I correct this? is their some configuration I need to do on the benefits to pro rate on the first period?

basically if the employee is hired in the middle of the payperiod the contributiosn are not calculated.

Any thoughts?

Thanks,

Vinu