In the Bartimaeus series, it seems that the quality of magic has declined. For instance, Khaba, in ancient Israel, could control a marid and several other spirits, the former of which alone modern-day Bartimaeus estimated would take two magicians. The Amulet of Samarkand has protective powers greater than anything modern magicians can muster up. Perhaps most astonishingly, the Ring of Solomon enslaved an entity of almost incalculable power, and it was ancient before Solomon's time. There are other, more minor examples of this trend.
All this despite the fact that magical investigation seems to have proceeded apace, with magical innovations such as spirit-based camera and such proceeding at a decent clip, and that the world population is a lot larger.
Even the staff of Gladstone, which is relatively new, is at least a century old, and it's the weakest of these major artifacts.
Why is this?