I've been getting some emails about sewing with elastic thread. It seems that when some of you try to make the smocked sundress from Weekend Sewing that you saw on the Martha Stewart Show or the grown-up "Mendocino" version, you are having trouble getting the elastic thread (in the bobbin) to stretch as you sew, which makes it impossible to get those nice stretchy gathered rows. Its tough for me to say exactly what the problem is in every case, because I can't see your machine or its bobbin. I can, however, tell you what might be causing the problem.
The bobbin, loaded with elastic thread, is often inserted int the machine incorrectly. Check your manual to make sure that you are putting your bobbin in in the right direction (this might not be causing you problems with regular thread), and that your thread is running through the proper path. Its it imperitive that your elastic thread be stretched as it sews. In a machine with a bobbin case this will occur as it is pulled through the opening in the side of the case, just as your regular thread would be. If your machine has a drop-in bobbin, your bobbin thread needs to be threaded carefully through the proper channels, which vary a little from machine to machine. Sometimes, an improperly loaded bobbin will only cause minor problems with regular thread, so you might think you've got it in the right spot until you try to sew with the elastic thread!
In rare cases, I have seen sewing machines just simply refuse to shirr.. My best guess is that the tensioning springs and discs are designed to accomodate many types of thread, and self regulate themselves so that the elastic thread is allowed to pass through without being stretched. I have been able, in most cases, to tighten the tension settings for either the bobbin or upper thread with successful results. Every machine is unique, so spend some time experimenting with some scrap fabric.
For a great piece on sewing machine tension, head over to the Threads Magazine site. I also love this little animation, (scroll down to see the lock stitch) which shows just how delicate and crucial proper thread tension settings are.
I hope this helps! -Heather