A few things aren't clear from your post, and the answers will fundamentally alter the advice.
Are you watching recordings on the same box as they're recorded on? Or are you streaming recordings between your 2 x V6? And can both boxes use OnDemand services reliably? If you're not sure - please check. Pick a programme from ITV, C4 or C5 and choose "watch from CatchUp" from the TV guide.
Each of your V6 will have 2 separate connections:
1: A co-ax cable feed carrying live TV channels
2: An internet connection, via your homehub, for OnDemand & all forms of streaming (multiroom & app-based). This connection can be wired or wireless - how are yours connected?
On any box that's connected via wireless, please follow Home > Help & Settings > Settings > Network, and post what's reported in the top-right. Especially the WiFi signal strength.
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