Generally you would think that cyanide, a highly potent drug, will efficiently kill a person. Especially with 7 doses, the target should be dead in no time. This is true, with one little exception that applied in this case:

Do not serve cyanide with wine.

The chemicals in wine act as a neutralising agent for cyanide, so along with giving Rasputin the poison, Felix Yuspusov was also giving him the antidote. Not a very smart thing to do, was it?