When printing your own heat transfer, never use a printer that has a self-drying mechanism installed. The heating process can and will ruin a transfer, resulting in the transfer not sticking to the shirt or cloth when pressed. You must use an ink jet printer for print-your-own heat transfers; laser printers will not work.
Hot peel transfers must be peeled off a shirt immediately after pressing. If a hot peel transfer is allowed to cool, it will cause the plastisol ink to stick to the paper it came on rather than the shirt. If a hot peel transfer cools down before you can peel it, simply press it again to heat it back up. Be sure to read the instructions that came with your heat transfers. Typically, a hot peel is heated to about 355 to 365 degrees Fahrenheit for about 15 to 20 seconds and then peeled off quickly.
Similar to a hot peel, if a cold peel is not peeled off the shirt at the right time, it will fail to stick to the cloth. A cold peel should be pressed for about 30 seconds at around 335 to 350 degrees Fahrenheit and then allowed to cool before peeling. As with all transfers, practice makes perfect, so it is best to try this out on a shirt you do not want or need before pressing the real one.
The age of a heat transfer will affect the transfer's ability to stick to a shirt. Vintage heat transfers often have issues with not sticking right. Unfortunately, there is nothing you can do about the age of a heat transfer except move on to the next one and try again.
Keep your heat transfers in a dry place. Moisture will easily damage any transfer, causing it not to stick properly. Use airtight bags and containers for best storage. Once you've successfully transferred the image, avoid washing the shirt for at least 24 hours to avoid moisture damage after the transfer.
Heat transfers require a certain amount of heat in order to successfully transfer. This varies by each brand and type of paper and ink used in the heat transfer. If your transfers came with instructions, follow the instructions for best transfer. If no instructions were included, heat your iron to around 350 degrees Fahrenheit and start practicing. Adjust the heat until you find the best temperature for transferring. It helps to place a piece of thin tag board or thick paper between the iron or heat press and the transfer when heating. This helps shield the transfer from over heating.