I find that difficulty quilting through the T-shirt design depends upon how thick or rubbery the design is. If I am going to quilt through the design I slow down. The rubbery design has a tendency to cling to the needle. I do not use a ball-point needle because I use regular quilting cotton on the back.
I usually use clear monofilament thread on the top and a thread to match the backing on the bottom. Most of the time I stitch in the ditch around each block to stabilize the blocks because I do not back my T-shirts with stabilizer. Then I go back and perform quilting in each block. I usually outline the design as well as I can and then perform some type of all over design in the rest of the block - flames, stippling, loop to loop, echo quilting, puzzle shapes- etc. I usually change the design from block to block.
Here is a link to one of the T-shirt quilts I have made.
http://www.quiltingboard.com/main-f1...ml#post4790296