If you don’t know me I am a Sewing Machine tech for over 30 years. Every now and then as I see the need I will post something to my blog
One of the biggest mistakes I see people making is adjusting machines with the mistaken assumption what is wrong. The main one is the machine is sewing with loops in the thread, there are big loops on the bottom so they tighten the bobbin case. This is the EXACT opposit of fixing the problem.. If you have big loops of thread on the bottom of the fabric you have no upper tension. Most likely it is misthreaded. If you are having problems rethread the machine carefully making sure the thread goes though all thread guides and the tension unit. If that does not fix it, it probably needs service but be sure the thread is between the tension disks and not behind or in front of them.
The only time you need to look at the bobbin case is the bobbin thread is pulling to the top. Most likely the bobbin thread is not in the tension and just pulling up. It is rare the bobbin case needs adjusting, if you need to make adjustments use the upper tension dial that is why it is there! It could be the upper tension is too tight if it is just pulling up a little, loosen the upper tension, lower numbers are lower tension. Typically most mechanical machines sew between 4-5.
Another problem I see is they misthreaded it, pulled the bobbin case out of position, possibly damaging it. Often they move the brackets so there is no play in the bobbin case so it cant come out again but this is wrong. There must be some clearance around the bobbin case for the thread to flow around it. If there is no room it will jam up.
Needles are another source of problems. If the needle is old it gets dull, it should not make a popping noise going in to the fabric. If you hit the plate because you are pulling the fabric or other wise dammage the needle it must be replaced. If you are breaking needles there is a problem. There are all kinds of types of needles, there are special Denim needles which are sharp, Knit needles for knit material, if your having skips on knit material like a t-shirt be sure you change to a knit needle.