Remember the items are sent by their stores, rather than a warehouse. I've noticed that when there's only 1 or 2 items in stock there's a chance that they will cancel the order if the store is unable to find the item.
According a member of staff in the Watford store, sometimes the delay is caused by lack of staff unable to pack your order.
Quite annoying that happens really. Surely if someone ordered it online the night before and a customer tried buying it the next day it should flag up on the till that it's been sold.
Have to admit when going into one of their stores there's nobody free to talk to. Was amazed how busy it is in there.