it can depend on the thread you use as well. Without adding more mind boggling things to think about when sewing with a machine and I am still learning myself, I have basic things that I look at when I get any whoopsies with the stitches.
Cotton - is it the correct one, I found the polyester sew it all stuff is marvellous, old cotton reels can get rotten and cause problems, is it threaded correctly in the machine, the amount of times I have put the reel on the wrong way.
Bobbin - is it the same type of cotton used on the top, wound properly, in properly, any damage to the bobbin, sometimes they can need replacing if needle damage has happened.
needle - is it sharp or bent, I change mine often (but then I use my machine a lot), is it the right one for the job - I use 90/14 sharps and I sew fleece a lot, any snags in it can cause the cottons to shred or snap, can create the problem you have photographed too as I have had that in the past.
Tension - generally is should be around the 4 or 5 on most modern machines, have put my domestic janome machine in for repairs as it was having terrible problems with the tension, would sew at 3 or 2 and that isnt right, turned out to be a timing problem in the end.
Servicing, simple servicing can be done like de fluffing the feed dogs (jagged bit under the metal plate), the bobbin casing and outside of this, a little oil in the center of the bobbin housing before putting it back together. I use a stiff artists paintbrush to de fluff my machines every week, but then again I do use it a lot.
hope that is of some help to you
