Me=Dunce

I broke my sewing machine I think. Can someone wiser than me figure out what I did? It all started when I tried to make a shirt for my cousin's daughter's birthday. My cousin is a major Elvis fan, so I wanted to sew a panel of this fabric onto a plain shirt for her daughter. I have a simple machine and the prettiest stitch I can make is a zig zag. Well...it started great. Til I realized I had sewn through both layers, front and back, of the shirt. DOH!





I removed the screwed up shirt and went and bought another and tried again but this time my zig wouldn't zag. The top stitching is working, but the under-stitching isn't. I probably am not using the right terms, sorry. Complete novice here.



Can anyone see what I have done wrong? I had my granny and mom, both experienced at sewing, and they don't see a problem with anything and can't figure out how to fix it. I keep rethreading it and rethreading it and it does it EVERY TIME. Also, the thread comes out oily. And my last project was a mei tai carrier. I think I tried to sew through too thick a set of layers and it bucked on me once or twice and since then, it's been acting ugly. But rethreading it always fixed it.