UHD Gone to Pot Again!!

Last week watching a match on Sky Sports on Sports UHD channel, the picture was fine. Now, it has gone back to what it was, dull, lifeless, grey looking. Really annoyed since UHD is part of my package and I am paying a hefty sum for the privilege. Have changed the HDMI Cable to a much faster one, TV is definitely 4K.  Anyone else enduring the same problems?