Have Avanti West coast stopped offering groupsave on tickets for 3-9 adults? I was trying to look some up for June but adding the groupsave discount doesn’t take any money off.
So far as I am aware Avanti West Coast have never offered GroupSave, nor did Virgin Trains. They have/had their own group discount facility that involved special Advance ticket types, though the discount is/was rather poor, and if applicable to your group Family Railcards tended to provide rather better value.
GroupSave was traditionally a London and South East commuter TOC thing, though it's available slightly more widely than that now, e.g. GWR do it on their whole network rather than just the former Thames Trains bit. Can you do your journey on LNR instead, perhaps? They offer it.