that could be -- your machine bobbin area needs to be cleaned out- a lint build up- or your needle needs to be replaced---or sometimes some threads just do not want to work in some machines. my viking is the pickiest machine i've ever owned---i can use a thread with great success on the long arm- use the same thread again with great success on the vintage singer---load that thread on the viking- and have nothing but aggrivation until i finally accept the fact the machine just does not like it! once i switch thread everything is good again-
but start with a cleaning- new needle- rethread & put the bobbin back in (sometimes trying a different thread in the bobbin helps too)
if all else fails- try a different thread- if that fixes it- then make note that thread does not work in that machine- and move on---sometimes there is just nothing else to do.