Stitches not forming!

Question

My machine is only about a week old but I'm already having problems. The stitches just aren't forming. I've tried adjusting the tension but it hasn't helped. I've re-threaded both threads, following the instructions very closely so I'm sure it's right. The top thread seems fine - goes down and comes back up in a loop, but the bottom thread doesn't form the stitch. I hope I'm describing this okay. Thank you!