This issue is likely due to not using enough water when sterilizing with your Philips Avent Premium 2-in-1 Warmer & Sterilizer.
Adding too little water makes sterilization finish too quickly, while adding too much water makes it take longer.
Fill the base up to the marking with exactly 60 ml/2 oz of fresh, room temperature water (see the image below) before every sterilizing session.
Note: Always add the correct amount of water as indicated and do not allow the base to overflow.