Shortfall occurs when a consumer’s currently allocated slots (in non-management resource groups) are fewer than the number of guaranteed slots and there is an unsatisfied demand on those resource groups. This could occur if you have too few slots available in the hosts belonging to your resource group and that resource group has too many consumers drawing from it. You should add more member hosts to an under-allocated resource group or assign more resource groups to the consumers involved.
You can quickly identify if this condition exists in either the cluster or in one branch of the tree.