I’ve had to unpick and start again on a curtain hem quite a few times now.
I’ve tried on my Singer 99k and Brother XL711. At first glance it looks like tension, because there is one line of thread lying flat top of the fabric. But on closer inspection, there are no loops coming up from below at all, so the stitches are not forming. If I lift the thread up, it comes away cleanly with no thread on it from the bobbin at all. It will go OK for a bit and then miss stitches again with no apparent cause. The only thing that I’m doing differently is that I’m using basting tape instead of pins. I have noticed that the needle shaft gets a bit gummy every now and then. When I tested for tension on a scrap of the same fabric (without basting tape), it was working OK, although admittedly I only sewed a few inches. Surely if basting tape caused this problem routinely, they wouldn’t be selling basting tape anymore, would they? Any thoughts?