If a jar will not hold a seal, it is almost never the machine. It is nearly always one of three small things, and they are all quick to fix. Work through them in this order.
First, how to test whether it actually sealed
Press the centre of the lid with a finger. A sealed lid feels solid and does not move. If it flexes under your finger, or pops back at you, the vacuum has not held.
The other test is to try lifting the jar by the lid alone, over the bench. A properly sealed lid will hold. Do this over a bench, not a tiled floor.
Cause 1: the rim was damp or dirty
This is the most common one by a wide margin, and it is the easiest to miss because a trace of flour or oil on the glass is nearly invisible.
The lid seals against the glass rim. Anything sitting between the two, a smear of oil from nuts, a dusting of flour, a drop of water from the dishwasher, gives the air a path back in.
Fix: wipe the rim with a dry cloth every single time before you put the lid on. Make it a habit rather than something you do when there is a problem.
Cause 2: the lid is the wrong size
A lid that is slightly too large will still sit on the jar. It will look fine. It will sit at an angle so small you will not notice, and it will never hold a vacuum.
Small lid for regular-mouth 70mm jars. Large lid for wide-mouth 86mm jars. If you are not certain which jars you have, our guide to wide-mouth versus regular-mouth jars sorts it in ten seconds.
Fix: put the lid on and look across it at eye level. It should sit flat and flush with the rim, not overhang the edge.
Cause 3: the sealer was not pressed fully down
The sealer has to seat properly on the lid before it can pull a vacuum. When it seats, you hear and feel a distinct clonk.
No clonk means no seal, every time.
Fix: sit the sealer centred on the lid and press down firmly until it clonks. Once it has, let go. You do not need to hold it there while it runs.
Two more worth checking
You stopped the cycle early. Let it run the full 40 to 60 seconds. Cutting it short is a common cause of a weak seal that fails overnight.
You pulled the sealer straight up. Lifting it vertically can drag the lid with it and break the seal you just made. Wiggle it gently side to side as you lift it away instead.
A seal that fails overnight
If the jar seals fine and has let go by morning, look at the rim of the jar itself. A chip or a nick in the glass, even a small one, means the lid cannot make a continuous contact ring. Run a fingertip around the rim. If you feel a catch, retire that jar to non-sealed duty.
The other overnight culprit is a warm, oily food sealed before it cooled. Let things come to room temperature first.
A note on liquids
FoodVault is designed for dry goods. Coffee, nuts, grains, flour, pasta, herbs and spices. It is not suitable for liquids, and trying will lead to a failed seal and a mess.
Still not working?
Send us a photo of your jar with the lid sitting on it, and a photo of the lid itself. Nine times out of ten we can spot the problem from the picture straight away, and it saves you guessing.
If you would like to run back through the whole process, here is how to use your jar sealer.