Hi. Its what the material is made up of
From Bella's website:
Bella 8800 Women's Flowy Racerback Tanks
Solid and Neon Colors*: 65% poly, 35% viscose, 32 single 3.7 oz. (Ath. Heather - 52% poly, 48% viscose)
Marble Colors: 91% poly, 9% Airlume combed and ring-spun cotton, 30 single 3.5 oz.
Stripe Colors: 70% rayon, 30% poly, 30 single 3.7 oz.
Slub Colors: 50% poly, 37.5% Airlume combed and ring-spun cotton, 12.5% rayon, 32 single 4.0 oz.
Black Slub: 40% poly, 30% Airlume combed and ring-spun cotton, 30% rayon, 32 single 4.0 oz.
Black Heather Flowy: 80% rayon, 20% polyester
Your not having issues with the Slub Colors...they will take on the heat your applying to it..the
solid colors will not. That's because the material content on the solid colors is 65%poly and 35%viscose...the material content on the Slub colors
is basically more towards a 50/50 combo.
You'll find that 50/50 and 100% cotton can take on
the heat (routinely 380f) without much issue whereas other materials might not. Unfortunately
there is no "guide" that I know of that will let you know what combos can take what heat. In your case you'll have to search out and find companies that produce a lo temp transfer that will work