Charles Ellicott Commentary 2 Samuel 20:5

"So Amasa went to call [the men of] Judah together; but he tarried longer than the set time which he had appointed him." — 2 Samuel 20:5 (ASV)

He delayed longer. —No cause is assigned for this, and various conjectures have been made. The simplest explanation may be drawn from the fact that, in 2 Samuel 20:8, Amasa is met on his return at Gibeon. He had therefore gone well beyond the boundaries of Judah into Benjamin, and had taken more time as a result of exceeding his instructions. This fact suggests great doubt about his suitability for the position David had promised him. Joab appears to insinuate (in 2 Samuel 20:11) that Amasa was not truly loyal.