Some abilities mention that hit targets may repeat a saving throw against a lasting effect to "shake it off". For example Command Undead:
If it fails the saving throw and has an Intelligence of 12 or higher, it can repeat the saving throw at the end of every hour until it succeeds and breaks free.
Others don't (for example, Channel Divinity: Turn Undead).
Are there general rules for which spells/conditions/abilities allow you to repeat their saving throws, or can you only repeat a saving throw if the condition/spell/ability explicitly says so?
I ask because I saw two spells which caused the target to be frightened. One mentioned that the target may repeat its saving throws the other didn't. It seems like a huge difference, but that may be because one of those spells is of a much higher level.
For another example, if you look at the Oath of Vengeance (PHB p. 88) you see that the Paladin's Avenging Angel and Channel Divinity - Abjure Enemy abilities each have a duration of 1 minute (or damage) and no repeating saving throws. Meanwhile almost every monster ability that can cause fear allows repeating saving throws. This makes it look like a mighty dragon is not as frightening to PCs as a paladin is to a pack of undead.