My vacuum flask no longer keeps warm, what has happened?Updated 2 months ago

It may be that your vacuum flask has lost its vacuum and therefore its insulating capacity. There is a simple way to check if this is the case. Fill the insulation bottle with hot water and feel the outside of the flask with your hands for a while. If the outside gets hot, it has lost its vacuum. This can happen if the bottle is knocked, dropped or otherwise punctured, but should not happen in normal use.