Is the decrease in profit in a shop, after it was transformed from a place of illicit practices to one that is permissible, a result of its previous corruption, and what are the ways to achieve abundant profit?

There is no Sharia-based evidence that links previous corruption in a shop to a lack of profit. Profit depends on both religious and material causes. The most important religious causes are Taqwa (piety) towards Allah Almighty, reliance on Him, and avoidance of sins. Faith and Taqwa bring blessings from the heavens and the earth, while sins obliterate blessings in one's life and sustenance. The questioner is advised to continue performing religious duties, repaying debts, and attending to material causes such as product quality and dealing with people of faith and trustworthiness. Furthermore, a smaller profit might be safer for his دين (religious commitment), as some people are corrupted by wealth. Let him remember the Hadith, "Look at those who are beneath you and do not look at those who are above you," and the Hadith, "Whoever is preoccupied with this world, Allah will scatter his affairs."