Price is agreed, terms are agreed, the wording being examined, and I imagine that the announcement will be made shortly.
However, it is possible that Arsenal are of the opinion that West Ham negotiated in bad faith - to my mind, their conduct during the negotiations (and I am not disputing their right to obtain the best possible price for their player) was less than exemplary, - and are aware that West Ham are on the FFP watchlist, (thus, they are unable to buy anyone until money due and owed is banked) and are deliberately dragging this out.
Neither team has covered themselves in glory with this.