Modern day pagan rituals and magic depend on ancient knowledge. This knowledge, however, is fractured due to the fact that most arcane knowledge from centuries past has been lost in wars, natural disasters, and the natural decay through time.
The most predominant pagan resources, say , Grecian and Roman, although vast, has been more or less damaged by the reign of the Roman Catholic empire. Before that, the Celts were wiped out by none other than the armies of Julius Caesar when he was only a general. In fact, the Shakespeare play baring that title, took place immediately after those invasions of the tribes of western Europe. Many of the Germanic and Celtic tribes passed down their traditions orally at those times, and when the Roman army went out of their way to kill priests and priestesses, most of those traditions were lost. We all know the story of the Native Americans and how they were raped and slaughtered nearly to the point of non-existence, they too, had a rich oral tradition. Although, some of the Native American traditions live on, I believe it is safe to say that, even on the reservations, the knowledge is fragmented and vague.
Heed this as you look through spells, and know that, sometimes, if the spell doesn't work, it's because we are all attempting to rewrite the untold history of the world, and the power that once could have been readily used is greatly reduced by the ancient wars of the past.
It is a sad thought, but it remains a fact. The winners of wars always are the ones to write the history books. Let your heart go out to the lost remnants of the past, and hope that they may bestow upon you, some of the knowledge that they once held.