Past sell by date

We are struggling to find Parmalat first growth 1+ for some time now. Yesterdag we finally got two boxes at PnP, only to find out last night, after giving it to baby, that it has gone bad, and when checking, also saw that it was past it expired date. One would assume that products would not be sold after experation date, and that shops would be able to give you some answers as to when stock would be available.