The post is a bit old but still;
Check points:
Needle in correct way, bobbin in correct way (flat side to the left),
Threading from right to left (if I remember correctly).
Bobbin in correct way, you just have to look at the manual, I get this wrong just going from one machine to another.
If problem persists, take the needle clamp completly off, scrape out the groove in the needle bar with a wooden tooth pick. After years of use dust gather there, any oil seeping down there makes it expan causing problems. It prevents the needle to slide all the way in and puts out of position to the hook.
For layers of thicker fabrics, flat felled seams and other humps and bumps you might up the needle size, it makes it easier.