Is undue delay or prejudice a sufficient basis for refusing to allow an amendment?

Appellants allege that the district court refused to allow another amendment because it was impatient with appellants' perceived delay. Appellants then claim that undue delay alone is an insufficient ground for refusing amendments, and that bad faith or prejudice is required. See United States v. Webb, 655 F.2d 977, 990 (9th Cir.1981). Appellants claim that appellees have not shown any prejudice and another amendment should be allowed.