issue when using customer exit during i-step =3

I have some code that checks if they user enters the correct value and sends back a message if they enter an incorrect value. The code works but when you enter a correct variable on the selection screen the message keeps popping up like it doens't see this new value. Is there another piece of code I need?

CASE I_VNAM.

IF I_STEP = 3 .

LOOP AT I_T_VAR_RANGE INTO LOC_VAR_RANGE

WHERE VNAM = 'ZP_ROLLDOCTOR' OR VNAM = 'ZP_TRIAL'.

ENDLOOP.

IF LOC_VAR_RANGE-LOW = 'Y' OR LOC_VAR_RANGE-LOW = ''.

SKIP.

ELSEIF LOC_VAR_RANGE-LOW = 'N'.

CALL FUNCTION 'RRMS_MESSAGE_HANDLING'

EXPORTING

I_CLASS = 'RSBBS'

I_TYPE = 'E'

I_NUMBER = '000'

I_MSGV1 = 'Must Enter Y OR leave blank .'

EXCEPTIONS

DUMMY = 1

OTHERS = 2.

RAISE NO_PROCESSING.

ENDIF.

ENDIF.

ENDCASE.