In chemistry in general, a lot of times, minute impurities can have a massive role on the properties of materials (this is especially apparent in food, were flavors are basically entirely defined by chemicals found in ppm quantity). Modern process control essentially works by having the needed and acceptable impurities measured in precise amounts, and relying on chemists and metallurgists to monitor how these impurities come about, but this kind of control has only really been possible for a century or maybe two.

In more ancient times, the process control instead basically relied on "get your ingredients from this well/mine/whatever". So while we actually have a decent idea of what Greek fire is (it's a kind of napalm), we don't have any way of knowing what their actual source of naphtha was, so we don't know what the precise impurities that gave it its best kick were. (I'll also opine that there were probably several different formulas in use, and we may be ascribing properties of different kinds of Greek fire to the same one, which would mean that the search for "the" formula for Greek fire could only ever be a failure.)