Probably if we think in the line why furan is less aromatic than pyrrole, we will get the answer.

A stock solution of 7.5 mg of fluorescamine in 25 ml of acetone is stable at room temperature, if kept free of moisture. Fluorescamine hydrolyzes quite rapidly in water to give non-fluorescent products. The half-life for the reaction with peptides is 10-100 milliseconds, with hydrolysis taking 1-10 seconds.

Spectrophotometry and Spectrofluorimetry: A Practical Approach, Ed., Gore, M., Oxford University Press, Incorporated (New York, NY 2000) p. 63.

It rapidly hydrolyzes in water yielding non-fluorescent products.