Dear Sailaja,
It seems you have joined in the company very recently, m i rit?
If yes,
1. First check with your senior consultant about this scenario or at least get some config stuff or valid information from respective persons.
2. First you should have to know for shift allowance either Z-Program or PCR are being used?
3. If it's PCR then what logic is written or If it's Z-program what logic is written? check both with concern persons don't be rush..!
4. You said from four months on-wards allowance isn't generated but before four months how did client handled...?
Note: Shift allowance is very common requirement from any client I hope so PCR might be written, Z-Program won't be written until very complex scenario...!
Hope above information is help to you....!
Ur's Mohan