Why hospital bombings remain difficult to prosecute as war crimes

Dear Editor
Why is it so hard to prosecute ?
Didn’t the German leaders in WWII get tried and found guilty of war crimes despite not having been actually in the concentration camps but because they were responsible for ordering the soldiers to do what they did. Can this same prosecution example be applied to the Russian military for what they are doing ?
Thanks for reading this
Regards
Joanne Dollimore