All fabric that HeatnBond is used on must be prewashed due to a chemical on it called sizing. It creates problems with the adhesion of the HeatnBond and could also be causing some of the stiffness you are experiencing because sizing actually stiffens fabric.
If the fabric is exceptionally thick and you have several layers it could also result in stiffness. Another possibility is that the fabric is quite thin and you are feeling the stiffness of the adhesive through it.
If this is a concern, you may want to try HeatnBond Soft Stretch for a lightweight iron-on web adhesive specially designed to move with your stretch fabric while maintaining a strong bond! There is no added bulk and maintains a soft hand after fusing.
For non-stretch fabrics such as quilting cottons, HeatnBond FeatherLite provides a virtually weightless bond that gets even softer with laundering. Like HeatnBond Lite, FeatherLite must be sewn to be permanent.
For more tips on how to best use our product, we recommend watching our tutorial videos for HeatnBond UltraHold, HeatnBond Lite and Featherlite, HeatnBond Hem Tape, and HeatnBond SoftStretch. These are valuable for first-time users to better understand the product and its applications as well as seasoned pros to refresh your technique and ensure you are getting the best results from our HeatnBond products.