Troubleshooting Bobbin Tension Issues with your Sewing Machine.
I don't really get tension problems when sewing stretch fabrics on my sewing machine (Brother XL-5021) but recently I've had a lot of issues when sewing woven fabrics.
These are the steps I usually take when troubleshooting and most of the time the problem is fixed. It's important to follow the guides in your manual for your particular machine though.
unplug the sewing machine
unthread the sewing machine
unscrew all the removable parts, including the foot plate and brush out the lint, etc
apply sewing machine oil to the moving parts (consult your manual!)
replace the needle
test sew on scrap fabric, adjusting the top tension until the stitches are well balanced
Recently I have found that the stitching on the underside of my sewing is loose and uneven. I thought this meant that the Upper Tension needed to be increased - however this made no difference.
I did an Internet search and came across superiorthreads.com a site specialising in helping quilting and sewing enthusiasts learn more about threads, needles, notions, sewing and quilting. There are some great tips here and it's well worth taking a look.
Superiorthreads.com recommends adjusting the Bobbin Tension on the bobbin case. This is something that you don't usually touch so take a photograph of it in its current position incase you want to return it to its original state.
Over time, tensions can change with regular use. Even though you haven't physically changed the settings, they can work themselves either tighter or looser. Thread, lint and even temperature can affect them.
Source: Superior Threads
To increase the tension in the bobbin I turned the screw, in small increments, in a clockwise direction, testing on scrap fabric after each adjustment. After a quarter turn I found that the stitches were beautifully even on the top and bottom of my fabric. This was such a relief as I was getting so frustrated with my sewing - when you go to the time and effort of making your own clothes you really want them to look professional and be well-made. Also, my machine is running beautifully now because it's also been cleaned and oiled!
If you've tried everything else and your tension is still not correct it may be worth giving this a try - but please check with your manual first!