All moulds must be elevated to allow air out from between the glass and mould.
This is not a big problem, as it will not create any problems, but it does show a lack of thinking about the mould itself.
There are some things you need to check.
Are the holes in the mould at last touchdown point(s)? Sometimes the vent holes in moulds are made at convenient points rather than at the places where the glass will last touch down in the mould. On a simple ball mould, a hole at the centre will be appropriate, as this is the last place the glass will touch. On a bowl with a square base, the last places the glass will touch are the corners, so that is where the holes need to be.
Are there holes in the side of mould to allow air out from under the mould? If there is one or more, there is no need to elevate the mould. The air will move out from under the mould through the hole in the side. In general, moulds are not so uniform on their base that they fit the shelf enough to seal the displaced and expanding air underneath the mould.
Are the holes clear? This is more important. If the vent holes are not open due to kiln wash or other things blocking the space, there will be no escape for the air. The vents need to be checked on each firing to ensure they are open.
Does the mould need holes at all? There are a number of shallow slumpers and other simple moulds, such as a wave mould or any cylindrical mould form, that to not need vent holes, either because they are so shallow, or because the air can escape along the length of the mould.