First, remove the seal and check that there is no damage to the inside of the lid or seal. If there is no damage, replace the Cook Expert glass lid seal checking that it is properly slotted in the rim.
If your lid is damaged in any way, contact Magimix Customer Care by completing the Contact Us form here.
Please note that if you are heating milk, reduce the amount (milk can easily boil over, just as it does in a saucepan).
Happy cooking!