Wouldn't they have written down how make it?
The formula for Greek Fire was a closely guarded state secret for the Byzantine Empire because of its effectiveness in battle, especially in naval warfare. As such, in order to keep the full process of how to make it secret from their Muslim and later Catholic rivals, it was made sure that absolutely no one person knew how to create the finished product from scratch.
Instead the recipe was broken up and given to different military engineers so that no one person knew the entire process should they be captured/defect. With each engineer working together however, they could create the formula for Greek Fire piece by piece while others could assemble the delivery methods such as the siphons the fire launched from.
This method of separating the formula so that no one person could create it without the organization of the state behind them was indeed a genius move that kept the weapon from being used against the Byzantines, but the drawback is that once the government breaks down in efficiency and ability to administrate, the state risks losing the whole formula itself, which is exactly what happened once Constantinople was sacked by Crusaders in 1204.
History gets a bit hazy here, but historical accounts of Greek Fire pretty much disappear after the crusader sack of the Byzantine Capital, indicating that the administration collapsed to the point where the successor Empire of Nicaea lost some or most of the engineers who knew components of the Greek Fire formula, losing the ability to produce it themselves by the time the Palaiologos dynasty recaptured Constantinople from the crusaders.
As for the formula itself, there's been many attempts to recreate it by engineers and chemists but so far we only have bits and pieces of the entire formula ourselves, an earlier answer by /u/sturbine listing ingredients such as a mixture of oil, bitumen, naphtha, resin and sulphur.
If you want to know more, I'd definitely check out reading A History of Greek Fire and Gunpowder by J.R. Partington for a more in-depth look at its history and potency on the battlefield.