Because she has Atypical Cushings and it was undiagnosed for a time (as it is w/ all Cushingoid dogs) - this means she had excess cortisol ('steroid') at all times in her system - this alone can cause weight gain and weight-holding and hunger. Since you're treating w/ the Lignans and Melatonin right now, it *could* be having some effect on the cortisol levels -- but yes, the only way to confirm that is to run another ACTH.
On the upside, this could be a VERY positive sign that the regimen could be working for her
!!!