Hello Chitra,
You need to satisfy the following criteria to appear for CBSE Compartment exam:
1-If yiu have failed any one of their subjects can appear for this exam.
2-In case,you appeared for six subjects during her boards in March and failed in two, and then she can appear for any of the two subjects.
3-If you appeared from six subjects were declared PASS but failed in one subject  they can appear in the Improvement category for this exam.
But once the re exam us conducted before, you are not eligible for the next time. So you have to give the exam of compartment at the time it is conducted.