Not calling my driver program for cheque printing

Dear experts ,

I am doing cheque printing using sap scripts.for that i have copied the F110_prenum_chck form and RFFOUS_c driver program. I made some modifications and assigned this driver program to my 'Z 'form.When i execute this application in debugging mode ,it's still calling standard drver program 'Rffous_c' only but not my 'z' drover program.what was the mistke i have done.Please help me out.

Thanks and regards

Naresh Bammidi.