I am afraid to admit to this since you all seem upset but....
i have worked for michaels and ill give you a run down of why this is happening... depending on what day you go (especially saturday late or sunday) there is an ad problem... be especially careful if you see tags that are different through the store... they are supposed to put up the new ad on saturday night and pull the old ad too... once the sign goes up, they will honor the sale price for you, BUT you must be watching for it because it will not register in the register system. this happens on sunday if there were sale signs that got missed by accident as well and are still up from the previous week.... always be sure to check as they are going through the register....
you could read the fine print on the sale sign too.... they alternate weeks... one week will have red letters on white, and the next it will be white letters on red..... once you discover the color scheme for the current week, it is easier to spot potential register problems.... your best bet to get through quick is to bring the old ad sign with you and your product to the checkout, that way, they know you found it on the sales floor...