This is likely because the needle takes some elastic fibres into the bobbin case. It is very likely that your needle needs to be replaced or you need to use a different type of needle.
Prior thread jams may also have left pieces of thread in the bobbin area so as well as changing the needle, take off the needle plate, take out the bobbin case to check. ( You may as well clean this area andl add a couple of drops of oil while you are at it.)
The Schmetz Universal needles have a slight ball point and can be used for both knitted and woven fabrics. So they are a good choice for general purpose sewing. They work for some, but not all, elastics.
If the Universal needle doesn't work, try the Schmetz Stretch needle. This needle has a rounder ball point, a long scarf to prevent skipped stitches and also has a non-stick coating to aid in penetrating "sticky" elastic fabrics.
Many sewers refuse to change a sewing machine needle until it breaks. A slightly bent needle will make a poor but workable stitch which is often blamed on the machine. I own a good quality sewing machine so the first thing I do if I have a stitch quality problem is to change the needle -- this fixes the problem 99% of the time.
PS There are likely other good brands of sewing machine needles as well as Schmetz. Use a brand recommended by a sewing machine repair person if you cannot find this brand. Some machines are forgiving with poor quality needles and some just refuse to work with them.
J Korte, April 2006
