I thought the vouchers arrived on Wednesday... A couple of times I've opened the app as soon as that, only to be greeted with "Vouchers are not ready yet..."
I haven't seen a treat end early as such, but there is a limited number of coupouns so there is a risk that trying to use one on a Monday or Tuesday could result in seeing "Sorry all the coupons have been issued". They may also have a daily limit, I'm sure I remember seeing that on the Mini Yule Log Bites quite early in the week window, and the entire shelf was empty of that line too in our Sainsburys so having a coupon wouldn't have mattered anyway, hahahah! I tried again on subsequent days during that offer but still no coupons available.
The person here who recently found the app saying that the coupon had been used when it hadn't, may have inadvertently activated the countdown on the scannable barcode - that's horribly easy to do. I keep meaning to screengrab the whole process because it's quite confusing. You have to click a button that says "ACTIVATE" I think just to get the process started at all, and you have to go through various steps that look like you're about to start the countdown just to get to the bit where you can actually see the list of products which are featured in the offer (not the photo of one with the edges of a few others visible behind it, but the full text list of every qualifying item) and that's ONE tap away from the countdown screen! Touch that final button and that's it, you've got 60min to use it and then it's gone. I'm not sure the app is smart enough to know whether you've used it or not, when the timer ends I think it considers the coupon "used" whether you successfully scanned it or not.
