Singer Sewing Machine Not Stitching Properly?

If you want to keep this machine, invest in a 'professional' to go over it for you. Sometimes a needle breaks and all pieces don't get removed, to wrong type of thread used, bobbin aged, esp if using metal bobbin. Keeping machine clean is very important, and oiled regularly WHERE it is required. Hope you have the manual for it, OR you can google MANUAL for said machine and model.

They are online, some free. I recently updated from my 30 year old machine. Had one from the 1960 prior to that. And yes, they moved to the basement for non-intricate sewing. They can still do tarps, heavy jackets, jean zippers, etc. Taking care of machines is what keeps them going.

When thread is bunching underneath the fabric it is usually a problem of tension balance. The upper thread doesn't go down enough. Reduce the tension on the needle thread by putting the tension knob on an average tension level.

There is also a little screw on the bobbin holder that adjust the tension on the bobbin thread you can tighten it with a screw driver. As Grandma J. said dusting, cleaning all parts
of the machine where the threads go through is very important specially the tension disks in your case (these are the disks the tension knob is pressing on) and of course make sure you are using the same kind of thread for the bobbin and to feed the needle.
They can be different in colours but they must be the same type of thread, cotton or polyester and good quality threads made for sewing machine and not thread for hand sewing which are wax coated to make them stronger.
If they are different in type it will be very difficult to balance the tension as for example cotton thread and polyester thread do not have the same elasticity.
Balancing the tension is the tricky part of sewing with a sewing machine even with the most modern machines. Once you have succeeded don't forget that the first thing you should sew is a proper cover made to the shape of your machine that will protect it from dust.

Hope this helps !