Without a photo it's hard to tell, however, I'm going to make a guess it's a stabilizer issue. Some embroidery designs are very dense. The whole design is completely filled, making it bullet proof. If the design is one of those, you need to make the correct choice on fabric and stabilizer. The denser the design, the more stabilizer you need and you will probably have better success with a heavier fabric.
When I took my first lesson with my embroidery machine, the most valuable thing the instructor told me was you need to learn to correctly match the design with the fabric weight. Basically don't choose a dense design for a lightweight cotton, or a t shirt. Yes, you can use a ton of stabilizer and get it done, but you probably won't be happy with the result, especially after laundering.
Here's a link to the place where I get my stabilizers. This is their education page. Scroll down to the stabilizer section and read the articles. Lots of good info.
http://www.allstitch.net/department/....cfm?killnav=1