Why was pressure building for a new constitution during the second half of the 1780s?

The reason is that the country was in a lot of debt.

Explanation:

America defeated Great Britain during the 1780s, won the battle, and set up a system of government. This composed of success, confrontation, political and economic turmoil and development. The American Revolutionary War concluded in 1783, with George Washington becoming America's first president elected in 1789.

After the battle, the nation had great debt and France just lent money at immense inflation. Paper currency has little value, and coins that caused a major economic recession were hard to obtain. The accumulation of debt and failure to pay debt contributed to revolts like Shay's uprising. For contrast, the United Kingdom declined to buy US goods.

While American colonists in the 1780s had trouble, if this decade of litigation, errors, progress and independence, the nation would not be that today.