I have found that washing a garment is sometimes necessary to remove the sizing in the fabrics, before blinging them. Also, sweatshirt fabric can be thick. So what I do, after I place the rhinestone onto the garment, I push it into the fabric with my finger, or the side of the hotfix applicator tool, placing one hand on the inside of the garment - pressing together. This gets the adhesive into the fibers rather than just on top of them. The stone should look like it's set into the fabric, not just siting on top if it. I use this technique weather I'm using hotfix crystals or flat back with an adhesive. When using hotfix crystals, I make sure the adhesive is fully melted before placing the crystals on the garment.
WHEN USING HOTFIX CRYSTALS - Sometimes, when I'm finished with my project, I'll turn it inside out, and use a hot iron and put pressure down onto the back side of the design. Iron set on cotton setting. Pressing for about 15 seconds or so - not sliding the iron around, but lifting it to move it around. Then let it completely cool before moving the garment.