Hi Kishore,
If you want to scramble only MATNR , there is a SAP delivered rule Rule_Material_Number which you can use as a reference (or) you can use the same rule for Scrambling. This rule has extra domains like MATNR_EXT , MATNR_INT.
Also even after cross verifying with this rule, if yours is still not working please create a message in component CA-TDM-SCR and I will look into it.
Best Regards,
Uma Tharmarajan