Refer to the maintenance section of your manual and clean the bobbin area and feed dogs to remove dirt, dust and lint. Oil as instructed applying only 1-2 drops each place (use only fresh, good quality sewing machine oil--NO old oil, cooking oil, 3-in-1, or WD-40). If instructions are to oil the race near the bobbin, do NOT skip this step as it is imperative for smooth operation of the bobbin and bobbincase.
When experiencing stitching issues, the first thing to always try is installing a brand-new needle making sure it is installed with the flat side facing the correct direction for your machine (see owner's manual). Use the proper size needle for your machine as there are several needle lengths available but not all will work in your machine (some sergers use a longer needle--very negligible but enough to cause the machine to malfunction). The needle and thread should be compatible with each other and with the fabric you are sewing. The following guide may be helpful in determining the needle to use:
http://www.schmetzneedles.com/all-about-needles/#sthash.rvoZCUwx.dpbs
The next thing to try is changing the thread you are using. If it is old thread, it could have deteriorated over time. Bargain bin thread is usually lower quality and could cause stitching problems as well as shedding excessive lint into your machine. Try using a better brand of thread like Mettler, Sulky, Aurafil, Gutterman, etc. NOTE: some machines do better with certain brands of thread, so experimentation could be beneficial in finding what works best for you.
Consult your owner's manual and confirm that you are threading in the proper order. Also, be sure to RAISE the presser foot BEFORE threading the upper thread as this releases the tensioner so the thread will seat properly. Set the upper tension guide at the mid-point (somewhere between the high and low number) as this is usually the factory setting. If the upper thread continues to be too tight, then try loosening it. If it continues to be too tight, the tension guides may need cleaning/service.
Verify that you are installing the bobbin correctly in the bobbincase and threading the bobbincase properly (when the thread is pulled, the bobbin should turn in a specified direction, in most machines it turns clockwise).
If you make it this far, you can try pulling the bobbin thread to the top of the fabric before beginning to stitch. This will help prevent thread nesting and upper thread breakage:
How and Why to Bring up the Bobbin Thread ...
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