craftgrrl | great fabrics and a little fabric dye help needed!

first thing, i'm a newb with fabrics, so i just get stuff that's nice and decently priced. anyways....
i found some wunnerful fabrics by Alexander Henry on ebay. he has alot of really fun stuff. so awesome i just had to share with you guys.
check out an ebay search of his stuff here. if you haven't heard of him, you'll like it!

secondly, i have a shirt that's like...i'm pretty sure mostly polyester and cotton..i think (forgive me i'm at work and not currently wearing said shirt :P) and i attempted to dye a bleach splotch on the left shoulder with Rit fabric dye. had to dye it twice for the dye to cover it up seamlessly. but alas, after a couple washes the bleach splotches are poping through again.

are synthetic(or at least half synthetic) materials hopeless to dye? or is there a better dye than Rit for that type of material?

any tips appreciated i've only ever dyed my hair before i attempted this, lol.