Very few players step up from "big fish in small pond" to "big fish in the biggest ocean" .... we keep expecting it to happen then are surprised when it doesn't. Partey is a decent player, with luck he will be a solid DM for the remainder of his contract, is he an EPL winning leader? There is nothing to suggest that so far. His game in Madrid was based around his ability to dominate an area of the pitch which, let's be honest, in 95% of Madrid games was under very little pressure ... that's just not the same in the EPL where almost without exception the midfield in 95% of Arsenal games is under enormous pressure, every poor touch, concentration fail, or bad decision is brutally exposed.
Partey is good, he could be very good, but right now he's still got a long way to go.