why medial eyebrow depression after 20u of Botox in glabella?

Medial brow ptosis or descent of the central brow can occur for several reasons:
Botox placed too low within the frontalis muscle along the forehead including central forehead.  The frontalis muscle elevates the forehead and by decreasing activity here the central brow can drop
Botox placed too superficially along the corrugator.  The botox will again impact the frontalis muscle and paradoxically lower the central brow.
Botox is best injected by an experienced injector to lessen wrinkles while improving facial appearance.

The vertical lines in the glabella are formed by the paired corrugator muscles, while the horizontal lines are created by the procerus.  Both of these muscles are brow depressors, meaning when they are treated with Botox the medial brow will elevate.  If you are observing medial brow depression after treatment of your glabella, the Botox was injected too high - thereby relaxing the frontalis muscle instead (the muscle which elevates the medial brow).  When the frontalis is treated, the medial brow will drop.  

It is uncommon for the eyebrow to droop after botox injection into the glabella.  If the frontalis, forehead muscle, was effected, then it is possible for the eyebrow to have difficulty moving upward.  It may be that your eyelid is drooping, this is possible.  Please go see the physician who treated you and explain your situation.  It is important to see a certified physician who is experienced in the future.  Regardless, any change in eyebrow shape or height will be reversed in 3 months.  I wish you the best of luck, Dr. Emer.