Why did the men who wrote the constitution feel that the government under the Articles of Confederation was not working?

Under the Articles of Confederation the government did not have to power to levy taxes or form an army. The Articles of Confederation did not give the government enough power to function. They could only ask for the support of the states and hope they complied.

In regard to taxes, nobody wanted to pay taxes, so the government had no money. Shay's Rebellion showed that the government also didn't have the power to protect the states should they need it.

The Articles of Confederation were too weak because they were written with they tyranny of England fresh in the minds of the authors. They wanted to limit the power of government as much as possible and consequently created a government that couldn't function.