This is mail order for you I think.
I've had great service from Star Fitness, but a strange mixed bag from Sportshoes.com
3 weeks ago I ordered a Concurve running top, it still has not arrived.
Last Friday I ordered a pair of running shoes and they arrived on Saturday morning.
I suspect that a lot of these companies dont carry a lot of physical stock, it would be hundred of thousands of quids worth of gear (and therefore money), sitting in a warehouse.
I guess they carry a minimum stock, and have agreed short lead times from their suppliers to satisfy unstocked items.
My guess is therefore that they are being let down on lead time by their supplier, but its only a hunch.