If you have ever pressed bags, onesies or other items that have a stitched seam or raised area you have probably found that your vinyl may not have pressed properly.
The issue is that the press is not fully coming into contact with the vinyl and material. To get a proper press you will need to raise the lower portion of the garment or bag to ensure you have proper heat and pressure against the press.
The solution to this problem is a teflon pillow. Putting this under or in the item to be pressed helps raise the target surface so that your heat press comes into contact with the vinyl as needed.