We are battling to get a vacuum on our lids after waterbathing our pestos. After filling and capping, we waterbath for 20mins at boiling temperature and then force the temperature down to 18 degrees as fast as possible without breaking the glass. There are invariably a few bottles with “poppy” lids after they have come back up to ambient temperature after cooling. It happens more in the summer months. We have tried two different cooling techniques. Moving product to different temperature baths, finishing with ice water. The other is using an aircon blowing cold air into a box containing the bottles. Both do the job to bring the temp down but we still get some poppy lids. These often result in oil blowing out of the bottle after a few days or weeks. Any clues? Many thanks.