A common problem in kilnforming is that the glass slumps into the mould unevenly. Several of reasons are given in this post about high temperature or fast slumps for uneven results.
There are two other things that can be done to alleviate uneven slumps.
Place the mould in the centre of the kiln to reduce any uneven heating of the glass. Uneven heating is a common cause of off-centre slumps. Where you have persistent uneven slumping with a mould it may be better to fire it on its own so the conditions can be best for it. Sometimes it is more economical to fire a single item rather than a crowded kiln shelf where the firing conditions must be for an average rather than the optimal firing schedule and conditions for one mould. Less of the resulting slumped glass is disappointing.
There is an alternative. Cut the glass so the fused piece will be slightly smaller than the mould top. This will allow the glass to sit inside the mould rather than on top. Frequently there is evidence of the glass hanging up on the side of a mould. Sometimes there are spikes where the glass stuck and stretched. (Another reason for Low and Slow)
A third method has been suggested, but I have not tried it. This is to lightly bevel the underside of the piece to be slumped. The basis for this suggestion is that a bevelled edge will fit the mould better by having a slope rather than a relatively sharp edge resting on the mould surface. I do know the other two suggestions work, but not this one, although it sounds logical.