Well, just like there are many different brands & weights of threads, sizes & types of needles there are opinions on what one should use. First make sure there is nothing wrong with your machine, that the bobbin & needles are in place correctly. I then suggest trying different weights of thread & needle sizes seeing which perform best on your machine. For some of the best information on thread & needles in one place, go to Superior Threads web site, everything they say will pertain to any brand, weight of thread & needle you may choose. Good luck in your search.
For me, I have a Baby Lock Ellisimo, I am currently using Superiors Bottom line in the top & bobbin. I also only use topstitch needles, also from Superior, with this current configuration I am getting perfect seams & much happier than I ever was with my Janome.