HI,
What function module you are using did any changes made in development system to that particular FM,Compare version management. This is because of conversion routine " CONVERSION_EXIT_MATN1_INPUT" " CONVERSION_EXIT_MATN1_OUTPUT" used before passing this input value to the function module. It automatically removes or adds zero before the material number.
Go to se11->type matnr in database table,click display=>Double click on data type it will shows the Domain,Double click it again you will get the convertion routines MATN1 for MATNR now double click it you can get the FM.
Use this 2 FM in SE37,Pass material number with zero and without zero you will get the difference.
Regards,
Ravi Shankar L