Is the reason prayers are not answered saying mockingly: "Allah will never answer their prayers," and how can one repent from that?

It is forbidden to say, "So-and-so is far from guidance," because it is a presumption against Allah. It has been authenticated in the Hadith that whoever says this, his deeds will be nullified, and it is obligatory upon the one who says it to repent. The fulfillment of supplication occurs in one of three ways: hastened in this world, stored for the Hereafter, or warding off an equivalent evil. Sins, in general, are a barrier to the fulfillment of supplication.