When adding iron on tags to clothes, the heat seal machine or iron melts the glue on the tag into the fabric. The glue needs a certain amount of heat and pressure to get a good seal, and not enough can leave the tag with a bad connection to the fabric. If you find that tags are not sticking well try higher heat, higher pressure, or both.
Fabrics that have some woven in stretch material like lycra or spandex can stress the glue's connection to the fabric. You can minimize the chance tags lose their hold by orienting the tag so it runs with the fabric's weave: Grab the fabric and see which direction it has more stretch, and which has less stretch. Then orient the tag so the long side goes along the less stretchy direction. This gives the tag the best chance to not be "stretched" into losing its hold over time.
Sheer fabrics have too smooth a surface for most iron on tags to stick. Lacy and loose weave fabrics don't offer enough surface area for a good hold. For these cases we recommend stitch-in tags.