Occasionally a dependent activity fails because of a temporary resource issue and that breaks the chain. There is no retry on error for activities not scheduled even though in a sense, they are scheduled by the parent. This requires someone in IT to discover the break, and restart it.
Not all of target data are updated as expected
An activity that is not scheduled cannot have Retry on Error set.
Resolving the problem
You can create a second copy of any activities that might fail this way, then conditionally launch them on error. DO NOT try to have the activity call itself on error. This could create a infinite loop that could bring the server down.