Did you make the Tshirts yourself?
I'm wondering if the knit in the premade shirt wasn't treated with some kind of finishing agent - especially a good brand of shirt. Did you wash it before trying to put the iron on on the shirt?
I've used any iron I had at the time, and no trouble. An old Canon inkjet and Rowenta, Sunbeam, whatever was around. No problem. The only difference - I made the shirts themselves and could put the decal on totally flat before sewing.