Is it regular sewing or FMQ? If regular sewing my machine will sometimes eat or birds nest when I sew triangles together. I have taught myself to start sewing 1/4 inch from the edge, back stitch to the corner and then proceed to sew forward. If it is FMQ it could be a number of things. It could be the thread, batting or fabric you are using. Try changing those one at a time and see if you can find the answer. Did they try it out at the store and it was working fine there? As others have mentioned, I don't leave the shop until I do a test run.