Ali, white bag with blue label, not a tile retail specialist.
Ahh that be a wickes product then?
The fact that it's not setting in-time or at all suggests to me that the bag has got a little damp, not surprising if it has come from wickes. But also thinking about it, as it's come from a high street DIY store, i can't see them paying particular attention to the dates etc on the bag. You could of had some old or out of date stock that's been slung on top of the new pallet.
If the bag has been stored incorrectly though and has become slightly damp this could activate the chemical reaction in the adhesive but be too dry to bond, if that makes sense? The bags weren't torn or split/damaged were they? From the reduced trolly?
I can't see the stock being that old that they're out of date. It sounds very suspect that the bag has become damp to me through incorrect storage and/or maybe tearing or damage to the bags.