Donner has a bit of a bald belly. Everyone else had or has tummy fur. After ruling out anything medical, baldness is not something I would ever treat for, beyond maybe a homeopathic remedy. Their baldness just adds to their charm and personality.
My big dogs get 1 Pepcid A/C or Tagamet about 15 minutes before their meds. Try to get something in the tummy with the meds. even if it just a few treats or a slice of bread.