What you are experiencing is rarely a major issue with the machine. A simple, step by step approach will probably have you and your machine getting along fine in no time.
Thread nests on the underside of your fabric are almost always the direct result of improper threading or a problem in the thread path or upper tension.
Let's first look at what is actually happening in the stitch formation process to cause these wads of thread. Ideally, the thread should form a neat coil (stitch) in the "center" of your fabric. When you get these thread wads on the underside, that is telling you that for some reason, the upper thread is NOT pulling up the bobbin thread and forming that snug, balanced stitch.
Here is the recommended check list for solving this problem:
2)Clean out the bobbin area, and feed dogs of all fuzz, thread and lint . Inspect your bobbin for burrs and defects and make certain that it is wound evenly. Check your bobbin tension.
To check bobbin tension, dangle the bobbin case over your other hand, grasping the thread near the case. A little wiggle/ jerk on the thread should cause the thread to pull out of the case simply by the weight of the bobbin. The bobbin case should drop smoothly and without resistance about an inch and half before stopping. If the bobbin does not move, the tension is too tight. If the bobbin drops quickly and too far, the tension is too loose.
3)Put in a fresh needle that is correct for the thread and fabric you are sewing.
4)Get out your manual.
5) Set your upper tension to the default (usually 3 or 4) setting.
6)Carefully and meticulously re-thread the entire machine. When threading the upper portion, MAKE SURE THE PRESSER FOOT IS UP until you get to threading the needle. If the foot is down when you thread, the upper thread will not seat properly in the tension discs, and you will get those dreaded thread nests/wads.
7))Be very careful to thread and install your bobbin correctly.
8)When beginning your sewing, hold on the thread tails for the first couple of stitches.
These steps are part of what would be found via the link I prvided in my earlier post.