What did those who favored centralization see as the most serious problem of the articles, and how would they have changed them? what had prevented these changes?

Those who favored centralization thought that the Articles of Confederation gave little power to the central government, and therefore the central government was unable to impose taxes, or execuse laws. They would have changed the articles so that they would have a stronger central government and levied taxes, but that was prevented because they needed a unanimous vote for that to pass, and that didn't happen.