There have been quite a few threads on here of Samsung TVs having issues with 4K/UHD content on Virgin, this can often be an issue with HDCP compatibility. To rule that out...
Check your V6 video output (Home > Help & Settings > Settings > Devices > Video output) - make sure that 1080 is selected at the very least. This can sometimes need a reboot to take effect.
Next thing - what is it that you're having issues with... is it live TV? OnDemand? Recorded programmes? Streaming apps?
Assuming you've ruled out the HDMI lead & port as both working, a video output self-test is part of the V6 boot sequence - so a reboot of the V6 with the TV connected & switched on, is a good test of whether you view the "starting up" cycle.
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Gone to BBC iPlayer on virgin box. No pictures, just the same.
Just gone to my programs, and for some reason the picture came on in the small box. Now picture on all channels. This is what happens regular. Could do this at least one a day. Tried leaving box turned on, but still happens. This is definitely a V6 problem, virgin should address this. It appears to be happening for a lot of people.
The issue keeps happening and can last for an hour or all day then will rectify itself. It is only happens on the 4K TV, tried other TV and runs fine. This is an issue with the Virgin box. I have contacted the team in the past. They have sent out a replacement box (not sure if it was a new one) still the same. They sent out an engineer and he changed the setting as mentioned in this feed. Still the same. It's getting very frustrating. Basically Virgin are providing a service / product that is not compatible with all TV units. Unfortunately I am in an area I have to have virgin.