used a monochrome image in coreldraw 16. when i publish to pdf in coredraw 16, all the monochrome image has become white box in pdf.

i used a monochrome image in coreldraw 16. when i publish to pdf in coredraw 16, all the monochrome image has become white box in pdf.. i used acrobat 6 compatibility while exporting. If i use acrobat 4 copatibility, the file is perfect...

naturally high version should support more option... any one help, why iam getting this problem in acrobat 6 or above compatibility...