Ironing labels that come off are often steam ironed or not pressed long enough. It is very important that you iron the labels without steam, with a very hot iron and press long enough. If the labels come loose: you can iron them on again, the glue on the back of the label melts and therefore only sticks with a hot iron, so there is still enough glue on the label to iron it on. Unless they are ironed on with steam the first time, the glue is mixed with steam.
Many tupperwares (especially from the Tupperware brand) have a very light relief so that the adhesive labels do not stick well after use in the dishwasher. You can check this by running your nail over the jar or lid. When you feel a relief, it is better to stick the label on the inside of the jar or lid, which is often smooth. Less hip but just as efficient.
You can safely iron the labels on fleece and other synthetic fabrics! You just have to make sure that the iron does not come into direct contact with the fabric: put a large handkerchief over the label and iron the label with the tip of the iron.
You can iron the iron-on labels on the fabric inside of sports shoes and gymnastics slippers. Eg on the inside of the tongue of the shoe.
You can iron the iron-on labels on the fabric inside of boots. This way you can also neatly name them.