What was the clear sign that the article of confederation didn’t work

"The weaknesses of the Articles of Confederation would quickly lead to problems that the Founding Fathers realized would not be fixable under the current form of government. Many of these issues were brought up during the Annapolis convention of 1786. These included the following: 

Each state only had one vote in Congress, regardless of size.Congress did not have the power to tax.Congress did not have the power to regulate foreign and interstate commerce.There was no executive branch to enforce any acts passed by Congress.There was no national court system or judicial branch.Amendments to the Articles of Confederation required a unanimous vote.Laws required a 9/13 majority to pass in Congress.States could levy tariffs on other states' goods."

The central government was very VERY weak.
So weak that it could not even collect taxes because states refused to pay them.

Word of mouth.