IT services - child changes do not change parent value

[walts65]

zabbix 3.2
If I discover I have an incorrect trigger as a child value, or I want to change a child entry, on changing that item it does not change the parent value.

i.e. I found 'network' availability was showing 40%. I had used an incorrect trigger (node5 say) as a child under network - but when changed (and node5 shows correct availability) 'network' still shows 40%

Is there a way of forcing a recalculation of the structure?

Thanks