Addressing Tension Issues are basically a series of adjustments, and understanding how to do so.
There are several factors that go into tension issues that arise while sewing. While frustrating, it's usually a simple fix once you understand how it's controlled.
Things that affect the tensions on your machine. Check in the order as listed...
Okay, so let's assume you've gone through this checklist and are confident that everything is threaded properly and ready to go. Take a sample of the same fabric that you are using for your project and test your tensions. Start sewing about 10-15 stitches or so, then stop and check out the top and bottom stitches....
How do they look?
*If the BOTTOM stitches are 'loose & loopy’, this means that the tension is too loose and an UPPER thread tension adjustment is needed. Turn right to increase the tension, about one 1 number at a time, and then try sewing again. If your machine is much older and there are no numbers on the assembly, simply turn the knob in small increments until the desired tension is achieved, testing the stitches as you adjust. NEVER TURN THE TENSION ASSEMBLY UP TOO MUCH AT ONCE. Repeat until the bottom stitches are no longer loose. Note: If increasing the tension does not correct the stitches, there may be an issue with your tension assembly.
*If the TOP stitches are loose (which rarely occurs), then this means that your bobbin case tension needs to be adjusted. Bobbin cases have a tiny screw. With a small screwdriver, turn the screw to the right to increase the tension JUST A HAIR. Then test sew a few stitches to see if it corrected the issue. In my personal experience it's been rare that I ever had to adjust bobbin case tension, but I have.
Note: if your top stitches remain loose no matter what you do, remove the upper spool of thread entirely and use a different spool. It means that the thread is not compatible with the needle or thread type that you are using.
! Also, make sure that the thread in the bobbin case and the spool you use are the same type and weight.
Other things to check:
Look at the fabric- How does it look? Is the fabric 'puckering' while you sew? If so, loosen the pressure regulating screw on the top of the machine by turning just a little to the left. When the fabric puckers, or bunches up somewhat, it means that too much pressure is being put on the fabric as it feeds. Just loosen the screw a little, test sew again, and keep adjusting the screw until the fabric no longer puckers. The fabric should remain flat and stay smooth while you are sewing.
Note: In some instances the upper thread tension assembly may be broken. Sometimes people might remove it and put it back without doing it properly. There's also a check spring in this assembly that sometimes breaks, is bent, or is inserted into the assembly wrong (If it has been removed). The check spring plays an important part in controlling the thread as it passes through it. Check to see if the spring still has a 'spring' to it, if it does, it's likely fine, if not, then it needs to be replaced; OR (if the tension assembly had been taken apart), it could be that it's put back together incorrectly. Luckily, check springs and complete upper thread tension assemblies are still available for most machines.
Basically that's it. Proper upper and lower threading, compatible thread, needle and fabric and a clean lubricated machine are all you need to sew with success.