The light on the on button should flash red if it has been held down for at least 3 seconds. If it does not flash red, it may be that the (breast) milk or water in the bottle is already at the correct temperature.
Another reason may be that the bottle contents are still too cold. If the food is 0 degrees or colder, it will not heat up. It is not possible to warm up frozen (breast) milk or frozen water in an S-bottle warmer; it will need to defrost before it can be heated up. Food kept in the fridge has an average temperature of 6 degrees.
If the red light comes on briefly then goes off again right away, this may indicate a fault. In this case, please contact Difrax Customer Services using the contact form.