M.I.Late

I don't know why this is happening. I was using one machine and switched to another to avoid the problem but it still is happening - even on a completely different machine.
My zig zag I'm using to outline/adhere an applique is showing top thread on the bottom and bobbin thread on top. It's only happening on the edges, but it seems random. I have a satin piece of fabric adhered with pellon to a fairly heavy interfacing. I have tried changing the tension, the needle etc. Not sure what to do now. I will add pictures, but could use some advice asap. Hope someone has the answers :-(