The vacuum seal on the food jar can be a very effective feature. This is what keeps the contents hot for so long.
Each food jar has a steam release button on the top of the internal lid. Normally a quick press on this button will release sufficient steam to release the vacuum and you can open your lid easily.
Occasionally the lid has been screwed on very tight and the vacuum is therefore extreme. In this instance, press this button and hold down for 30 seconds after which time the vacuum will be released sufficiently to open the lid. Repeat if necessary.