Automatic Plan Enrolments using FF and Eligibility Profile.

Summary

The Client has a leave plan which has eligibility criteria based on DFF on assignment. I have a FF attached to eligibility profile and if I run update plan enrollment it does enrollment as expected.

Content

A client has a leave plan which has eligibility criteria based on DFF on assignment. I have a FF attached to eligibility profile and if I run update plan enrollment it does enrollment as expected. But the expectation is whenever employee assignment is updated/corrected and DFF value changes, the employee should be enrolled in the plan or enrollment should end date. 

Why does this not work when on hire and termination plan enrollment happens automatically?