A configuration can arise where one or more agents look at (query) another agent. Alternatively, an action might trigger a note to be moved, in turn triggering an $OnAdd and so on.
When using 'Update now' from the File menu, all agents are fired once (in $OutlineOrder sequence) as are any action events dependent on the update cycle. In a cascading scenario, it may thus be necessary to call 'Update now' several times to flush an action through to completion, especially if using linked agents where the order of dependence does not follow $OutlineOrder. For instance, if agent C looks at agent B which looks at agent A, a change in A will need 2 updates to flush through to C.
Therefore, if using documents with cascading effects, be aware that a single forced update may not always 'complete' all actions as the user might intuit would occur from a single invocation.
Forced invocation of updates runs all running agents, i.e. regardless of priority setting unless the agent is turned off.