No, you must never skip a shake! Although you might not feel hungry, you need the nutrients. If you miss a shake you may well stop losing weight or gain as your body will not be taking in enough energy to function with. Eating under 500 calories a day is drastic enough, hence controlled by pharmacy or doctors, to go below that is dangerous.
I'm sure someone who can explain the scientific stuff will be along to explain more clearly.
Perhaps try drinking shakes earlier in the day, or drinking some water,cleaning your teeth/ having a breath strip straight after.
Good luck