For many transfers, you need to make sure you iron long enough. Even though the transfer releases from the paper it might not adhere to your fabric. While some brands may be better than others, I have honestly not found a brand that will not work.
For the ones you have already done, get some white tissue paper (the kind that you use for gift bags is fine) and place it over the design. Iron over that with the iron at the hottest setting for the fabric. Iron over that evenly in a pattern that makes sure that you iron all spots evenly. I usually start and end by ironing the edges and then do a zig zag pattern over the rest. Iron for at least 3 minutes, longer if you don't feel that is working. Then let the image cool before lifting the tissue paper. Oh, and the tissue paper will have a shiney and matte side. Put the shiney side down.
After the image is on there good (and you can repeat the above as many times as necessary), then wash your shirt in a vinegar wash: 1 cup vinegar in the washer only, nothing else, just the shirt and vinegar, no detergent no fabric softener. Then dry on a low heat setting. Not no heat, it needs some heat. If you only have one shirt, put a towel or two in the dryer to keep the shirt from bunching up.
Hope this helps.
