Employee pension rate not returned in Retropay

We are running a Retropay covering a 3 month period for an Employee who has a 'Teachers Pension Scheme Defined Benefit' element entry.

The employee pension rate is returned by the Calculator formula in Month 1 but not in the later 2 months.

The code in the formula is shown below, any idea what the issue is please?

CHANGE_CONTEXTS(PART_NAME = 'GB_PS_TPS_EE_PENSION_RATES')

(

call_formula('CALL_CALC_VALUE',

FLOOR(l_gb_annual_pen_earnings) > 'base',

l_gb_ee_pen_rate < 'ded_amount' default 0

)

l_gb_pen_ee_cont_derived = l_gb_pen_earnings * (l_gb_ee_pen_rate)/100