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.

These lines in between eyebrows appear because of the action of three powerful muscles. Sometimes those lines get very deep and Botox won't be enough. Then fillers will be needed. 

Botox is amazing for the lines around the forehead and glabella area.  It sounds like you have a ptosis or droop from the injection being injected too low in the forehead.  The droop will resolve completely but may take up to three months.

Most likely some of the Botox was injected into the forehead muscle which elevates the Brow causing your droopiness.  This will go away eventually. 

If Botox was only placed in the glabella, it is difficult to explain a medial eyebrow droop after injection. It is possible that some Botox reached the lower forehead muscle (frontalis), either intentionally or unintentionally, from Botox spread. If you rely on your frontalis to keep your eyebrow position, a heavy brow may develop. Another possibility is that your eyelid is drooping. This can occur after glabellar injections, and results from placing Botox to close to the orbital rim. Like any Botox effect, in either case, the droop is temporary and will improve in the next few months. In the future, make sure to see an experienced injector to ensure safe and precise injection.