There were a number of serious deficiencies to the Articles of Confederation that made it completely ineffective for governing the new nation of the United States. I will briefly discuss some of these below.
First of all, the Articles of Confederation created a very weak national government. There was no executive power. The national government could not collect taxes and instead relied on the states to voluntarily fund it.
Furthermore, there was no independent national system of courts. Instead, the national government had to rely on the state courts to set judicial precedent. In fact, state courts could overturn or simply ignore federal law in their individual jurisdictions. As a result, there was little the national government could do to enforce its laws in the states.
The Articles of Confederation gave Congress the power to declare war but left it no means to raise a military. Because it was up to the state militias to provide a fighting force, it was a slow and clumsy process to mobilize the military in the face of threats.
It was also very difficult to pass laws under the Articles of Confederation. Nine of the thirteen states were required to pass laws. That meant that a minority of states could easily block the passage of legislation that the majority of states favored. Since each state had just one vote in this process regardless of population, a small portion of the country's total population could prevent the passage of useful legislation.
Indeed, it would have been very hard to fix the Articles of Confederation piecemeal. The amendment process required the approval of every state. Considering the diversity of the country, it would be nearly impossible for every state to agree on an amendment. Therefore, once the other failures of this system were made clear, it was decided to abandon the Articles of Confederation completely and create a "more perfect" system under the US Constitution.