The US government under the Articles of Confederation lacked many important powers. Some of these were specific powers but others were more general.
Generally, the national government simply looked weak. It had no chief executive and no judiciary. This meant that the national government had no power to enforce or even to interpret laws that it passed. This made it weak in general ways.
The government also lacked several specific powers. It lacked the power to regulate trade between the states or between the states and foreign governments. This meant that trade wars could break out between states. It also meant that the US could not create any trade policies with foreign countries. When the British closed their ports to American shipping, for example, the US could not retaliate.
Most importantly, however, the national government lacked the power to tax. It could only ask the states for money and hope that they complied. This meant that the national government could not pay its debts (from the Revolutionary War) or even maintain a military to defend itself.
These weaknesses made it clear to many that a new constitution was needed.