I appreciate what you're trying to do with your "Price Promise". I quite like the fact that you care enough to check whether my shopping would be cheaper elsewhere, and give me a voucher back for the difference.
However, there's a small problem.
See, what you're actually doing is reminding me, week after week, that I would be better off shopping elsewhere. It hasn't escaped my attention that I have received several vouchers for money back and very few indicating that I actually saved. And while it doesn't matter so much as long as the Price Promise holds (since we get the vouchers), there will no doubt come a day when you'll stop this promotion. Knowing that, and putting those two facts together, what do you think I'll do?
So I'm sorry, but this particular move was not your best. You've gone and shone a great big spotlight on one of your key weaknesses, and in doing so have created discontent where before there was none.
Of course, there is a way out. And it's probably your only way out at this point. Basically, you need to lower your prices, right across the board of your "comparable items". That way, where just now I get a voucher back almost every week, instead I'll get more and more notifications that I've saved.
But good luck with that - Asda are rightly proud of their "lowest priced supermarket" tag, and I'd wager they will fight to keep them.