Sometimes the ambient temperature can be too warm or the friction of the churning and shaking process can create some warmth.
Find the coolest place in the house to leave the cream to culture. If the cream is too hot when it is ready to shake, cool it in the fridge for a couple of hours.
If you see any buttermilk after shaking for approximately 2 minutes, cool in the fridge overnight and whip in a blender the next morning.