In his final letter Jekyll explains what went wrong. Why couldn't he control Hyde any longer?

In his final days and hours, Jekyll explains that Hyde grew increasingly stronger as Jekyll began to fade away. To make matters worse, Jekyll's supply of potion salt was running out. He ordered more, only to discover that the new salt was not effective. After ordering the most pure salt possible, Jekyll finally realized that the original order must have contained an unknown impurity that was actually the key potion ingredient. Without any more of the original salt, there was no way for Jekyll to discover what that secret ingredient was. Jekyll realized he had no choice but to transform permanently in to Hyde.