The comment #4 has a lot of merit in it. It is very important to clean the lint and stuff out of the bobbin area and any other area one can reach into, and to change needles frequently.......along with not trying to race with the speed. Whether the machine is a "cheap" one or a very high end one, abuse will destroy it.........and knowing its limitations also aids in its working ability. Most of the time the reason for a break down is operator abuse/misuse.
As an aside....why do all machines cheap and expensive offer so many styles of buttonholes...I guess since I do not any longer sew clothing, that seems like overkill. When I did garment sew-and that was for personal use only-I recall only using one or two different kinds of buttonholes....actually the same one only different sizes......all those buttonhole stitches always amaze me on the sewing machines...that is not what I look for when purchasing one........